The previous chapter introduced the major top-level mechanisms in MySQL++. Now we’ll dig down a little deeper and get into real examples. We start off with the basics that every MySQL++ program will have to deal with, then work up to more complex topics that are still widely interesting. You can stop reading the manual after this chapter and still get a lot out of MySQL++, ignoring the more advanced parts we present in later chapters.
All of the examples are complete running programs. If you
    built the library from source, the examples should have been built
    as well. If you use RPMs instead, the example programs’ source
    code and a simplified Makefile are in the
    mysql++-devel package. They are typically
    installed in
    /usr/share/doc/mysql++-devel-*/examples, but it
    can vary on different Linuxes.
Before you get started, please read through any of the
    README*.txt files included with the MySQL++
    distribution that are relevant to your platform. We won’t
    repeat all of that here.
Most of the examples require a test database, created by
    resetdb. You can run it like so:
resetdb [-s server_addr] [-u user] [-p password]
Actually, there’s a problem with that. It assumes that the MySQL++ library is already installed in a directory that the operating system’s dynamic linker can find. (MySQL++ is almost never built statically.) Unless you’re installing from RPMs, you’ve had to build the library from source, and you should run at least a few of the examples before installing the library to be sure it’s working correctly. Since your operating system’s dynamic linkage system can’t find the MySQL++ libraries without help until they’re installed, we’ve created a few helper scripts to help run the examples.
MySQL++ comes with the exrun shell script
    for Unixy systems, and the exrun.bat batch file
    for Windows. You pass the example program and its arguments to the
    exrun helper, which sets up the library search
    path so that it will use the as-yet uninstalled version of the
    MySQL++ library in preference to any other on your system:
./exrun resetdb [-s server_addr] [-u user] [-p password]
That’s the typical form for a Unixy system. You leave
    off the ./ bit on Windows. You can leave it
    off on a Unixy system, too, if you have .
    in your PATH. (Not a recommendation, just
    an observation.)
All of the program arguments are optional.
If you don’t give -s,
    the underlying MySQL C API (a.k.a. Connector/C)
    assumes the server is on the local machine. It chooses
    one of several different IPC options based on the platform
    configuration. There are many different forms you can give
    as server_addr with -s to
    override this default behavior:
localhost — this is the default; it doesn’t buy you anything
On Windows, a simple period tells the underlying MySQL C API to use named pipes, if it’s available.
172.20.0.252:12345 — this would
        connect to IP address
        172.20.0.252 on TCP port
        12345.
my.server.name:svc_name — this
        would first look up TCP service name
        svc_name in your system’s
        network services database (/etc/services on
        Unixy systems, and something like
        c:\windows\system32\drivers\etc\services on
        modern Windows variants). If it finds an entry for the service,
        it then tries to connect to that port on the domain name
        given.
For the TCP forms, you can mix names and numbers for the host and port/service parts in any combination. If the server name doesn’t contain a colon, it uses the default port, 3306.
If you don’t give -u, it assumes your
    user name on the database server is the same as your login name on
    the local machine.
If you don’t give -p, it will assume
    the MySQL user doesn’t have a password. (One hopes this
    isn’t the case...)
When running resetdb, the user name needs
    to be for an account with permission to create the test database.
    Once the database is created, you can use any account when running
    the other examples that has DELETE, INSERT, SELECT and UPDATE
    permissions for the test database. The MySQL root user can do all
    this, of course, but you might want to set up a separate user,
    having only the permissions necessary to work with the test
    database:
CREATE USER mysqlpp_test@'%' IDENTIFIED BY ’nunyabinness'; GRANT ALL PRIVILEGES ON mysql_cpp_data.* TO mysqlpp_test@'%';
You could then create the sample database with the following command:
./exrun resetdb -u mysqlpp_test -p nunyabinness
(Again, leave off the ./ bit on Windows.)
You may have to re-run resetdb after
    running some of the other examples, as they change the
    database.
See README-examples.txt for more
    details on running the examples.
The following example demonstrates how to open a connection,
    execute a simple query, and display the results. This is
    examples/simple1.cpp:
#include "cmdline.h"
#include "printdata.h"
#include <mysql++.h>
#include <iostream>
#include <iomanip>
using namespace std;
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    // Connect to the sample database.
    mysqlpp::Connection conn(false);
    if (conn.connect(mysqlpp::examples::db_name, cmdline.server(),
            cmdline.user(), cmdline.pass())) {
        // Retrieve a subset of the sample stock table set up by resetdb
        // and display it.
        mysqlpp::Query query = conn.query("select item from stock");
        if (mysqlpp::StoreQueryResult res = query.store()) {
            cout << "We have:" << endl;
            mysqlpp::StoreQueryResult::const_iterator it;
            for (it = res.begin(); it != res.end(); ++it) {
                mysqlpp::Row row = *it;
                cout << '\t' << row[0] << endl;
            }
        }
        else {
            cerr << "Failed to get item list: " << query.error() << endl;
            return 1;
        }
        return 0;
    }
    else {
        cerr << "DB connection failed: " << conn.error() << endl;
        return 1;
    }
}
This example simply gets the entire "item" column from the example table, and prints those values out.
Notice that MySQL++’s StoreQueryResult derives from
    std::vector, and Row provides an interface that makes it a
    vector work-alike. This means you can access
    elements with subscript notation, walk through them with iterators,
    run STL algorithms on them, etc.
Row provides a little more in this area
    than a plain old vector: you can also access
    fields by name using subscript notation.
The only thing that isn’t explicit in the code above is
    that we delegate command line argument parsing to
    parse_command_line() in the
    excommon module. This function exists to give
    the examples a consistent interface, not to hide important details.
    You can treat it like a black box: it takes argc
    and argv as inputs and sends back database
    connection parameters.
The simple1 example above was pretty
    trivial. Let’s get a little deeper. Here is
    examples/simple2.cpp:
#include "cmdline.h"
#include "printdata.h"
#include <mysql++.h>
#include <iostream>
#include <iomanip>
using namespace std;
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    // Connect to the sample database.
    mysqlpp::Connection conn(false);
    if (conn.connect(mysqlpp::examples::db_name, cmdline.server(),
            cmdline.user(), cmdline.pass())) {
        // Retrieve the sample stock table set up by resetdb
        mysqlpp::Query query = conn.query("select * from stock");
        mysqlpp::StoreQueryResult res = query.store();
        // Display results
        if (res) {
            // Display header
            cout.setf(ios::left);
            cout << setw(31) << "Item" <<
                    setw(10) << "Num" <<
                    setw(10) << "Weight" <<
                    setw(10) << "Price" <<
                    "Date" << endl << endl;
            // Get each row in result set, and print its contents
            for (size_t i = 0; i < res.num_rows(); ++i) {
                cout << setw(30) << res[i]["item"] << ' ' <<
                        setw(9) << res[i]["num"] << ' ' <<
                        setw(9) << res[i]["weight"] << ' ' <<
                        setw(9) << res[i]["price"] << ' ' <<
                        setw(9) << res[i]["sdate"] <<
                        endl;
            }
        }
        else {
            cerr << "Failed to get stock table: " << query.error() << endl;
            return 1;
        }
        return 0;
    }
    else {
        cerr << "DB connection failed: " << conn.error() << endl;
        return 1;
    }
}
The main point of this example is that we’re accessing fields in the row objects by name, instead of index. This is slower, but obviously clearer. We’re also printing out the entire table, not just one column.
By default, MySQL++ uses exceptions to signal errors.
    We’ve been suppressing this in all the examples so far by
    passing false to
    Connection’s constructor. This kept
    these early examples simple at the cost of some flexibility and
    power in error handling. In a real program, we recommend that you
    leave exceptions enabled. You do this by either using the default
    Connection constructor, or by using the
    create-and-connect constructor.
All of MySQL++’s custom exceptions
    derive from a common base class, Exception. That in turn derives from Standard C++’s
    std::exception class. Since the library
    can indirectly cause exceptions to come from the Standard
    C++ Library, it’s possible to catch all exceptions from
    MySQL++ by just catching std::exception.
    However, it’s better to have individual catch blocks
    for each of the concrete exception types that you expect, and
    add a handler for either Exception
    or std::exception to act as a
    “catch-all” for unexpected exceptions.
When exceptions are suppressed, MySQL++ signals errors
    by returning either an error code or an object that tests
    as false, or by setting an error flag on the object. Classes
    that allow you to suppress exceptions derive from the OptionalExceptions interface. When
    an OptionalExceptions derivative
    creates another object that also derives from this interface,
    it passes on its exception flag. Since everything flows from
    the Connection object, disabling
    exceptions on it at the start of the program disables all optional
    exceptions. This is why passing false for the
    Connection constructor’s “throw
    exceptions” parameter suppresses all optional exceptions
    in the simple[1-3] examples. It keeps them,
    well, simple.
This exception suppression mechanism is quite granular.
    It’s possible to leave exceptions enabled most of the time,
    but suppress them in sections of the code where they aren’t
    helpful. To do this, put the section of code that you want to not
    throw exceptions inside a block, and create a NoExceptions object at the top of that block. When created,
    it saves the exception flag of the
    OptionalExceptions derivative you pass to it,
    and then disables exceptions on it. When the
    NoExceptions object goes out of scope at the
    end of the block, it restores the exceptions flag to its previous
    state:
mysqlpp::Connection con; // default ctor, so exceptions enabled
{
  mysqlpp::NoExceptions ne(con);
  if (!con.select_db("a_db_that_might_not_exist_yet")) {
    // Our DB doesn’t exist yet, so create and select it here; no need
    // to push handling of this case way off in an exception handler.
  }
}When one OptionalExceptions derivative
    passes its exceptions flag to another such object, it is only
    passing a copy; the two objects’ flags operate independently.
    There’s no way to globally enable or disable this flag on
    existing objects in a single call. If you’re using the
    NoExceptions feature and you’re
    still seeing optional exceptions thrown, you disabled exceptions
    on the wrong object. The exception thrower could be unrelated to
    the object you disabled exceptions on, it could be its parent,
    or it could be a child created before you disabled optional
    exceptions.
MySQL++ throws some exceptions unconditionally:
MySQL++ checks array indices,
      always.  For instance, if your code said
      “row[21]” on a
      row containing only 5 fields, you’d get a
      BadIndex exception. If you
      say “row["fred"]”
      on a row without a “fred” field, you get
      a BadFieldName exception. In
      the past, MySQL++ delegated some of its index checking
      to the STL containers underpinning it, so you could get
      std::range_error instead. As of MySQL++
      v3.0.7, this should no longer happen, but there may be instances
      where it still does.
String will always throw BadConversion when you ask it to do an improper type conversion. For example, you’ll get an exception if you try to convert “1.25” to int, but not when you convert “1.00” to int. In the latter case, MySQL++ knows that it can safely throw away the fractional part.
If you use template queries and don’t pass
      enough parameters when instantiating the template,
      Query will throw a BadParamCount exception.
If you use a C++ data type in a query
      that MySQL++ doesn’t know to convert to SQL, MySQL++
      will throw a TypeLookupFailed
      exception. It typically happens with Section 5, “Specialized SQL Structures”,
      especially when using data types other than the ones defined
      in lib/sql_types.h.
It’s educational to modify the examples to force
    exceptions. For instance, misspell a field name, use an out-of-range
    index, or change a type to force a String
    conversion error.
SQL syntax often requires certain data to be quoted. Consider this query:
SELECT * FROM stock WHERE item = 'Hotdog Buns'
Because the string “Hotdog Buns” contains a space, it must be quoted. With MySQL++, you don’t have to add these quote marks manually:
string s = "Hotdog Buns"; query << "SELECT * FROM stock WHERE item = " << quote_only << s;
That code produces the same query string as in the previous example. We used the MySQL++ quote_only manipulator, which causes single quotes to be added around the next item inserted into the stream. This works for any type of data that can be converted to MySQL++’s SQLTypeAdapter type, plus the Set template. SSQLS also uses these manipulators internally.
Quoting is pretty simple, but SQL syntax also often requires that certain characters be “escaped”. Imagine if the string in the previous example was “Frank’s Brand Hotdog Buns” instead. The resulting query would be:
SELECT * FROM stock WHERE item = 'Frank's Brand Hotdog Buns'
That’s not valid SQL syntax. The correct syntax is:
SELECT * FROM stock WHERE item = 'Frank''s Brand Hotdog Buns'
As you might expect, MySQL++ provides that feature, too, through its escape manipulator. But here, we want both quoting and escaping. That brings us to the most widely useful manipulator:
string s = "Frank’s Brand Hotdog Buns"; query << "SELECT * FROM stock WHERE item = " << quote << s;
The quote manipulator both quotes strings and escapes any characters that are special in SQL.
MySQL++ provides other manipulators as well. See the manip.h page in the reference manual.
It’s important to realize that MySQL++’s quoting
    and escaping mechanism is type-aware. Manipulators have no effect
    unless you insert the manipulator into a
    Query or SQLQueryParms stream.
    [2] Also, values are only quoted and/or
    escaped if they are of a data type that may need it. For example,
    Date must be quoted but
    never needs to be escaped, and integer types need neither quoting
    nor escaping. Manipulators are suggestions to the library, not
    commands: MySQL++ will ignore these suggestions if it knows it
    won’t result in syntactically-incorrect SQL.
It’s also important to realize that quoting and escaping
    in Query streams and template queries is
    never implicit.[3] You must use
    manipulators and template query flags as necessary to tell MySQL++
    where quoting and escaping is necessary. It would be nice if MySQL++
    could do quoting and escaping implicitly based on data type, but
    this isn’t possible in all cases.[4] Since
    MySQL++ can’t reliably guess when quoting and escaping is
    appropriate, and the programmer doesn’t need
    to[5], MySQL++ makes you
    tell it.
The C++ and SQL data type systems have several differences that can cause problems when using MySQL++, or any other SQL based system, for that matter.
Most of the data types you can store in a SQL database are either numbers or text strings. If you’re only looking at the data going between the database server and your application, there aren’t even numbers: SQL is a textual language, so numbers and everything else gets transferred between the client and the database server in text string form.[6] Consequently, MySQL++ has a lot of special support for text strings, and can translate to several C++ numeric data types transparently.
Some people worry that this translation via an intermediate string form will cause data loss. Obviously the text string data types are immune from problems in this regard. We’re also confident that MySQL++ translates BLOB and integer data types losslessly.
The biggest worry is with floating-point numbers. (The FLOAT and DOUBLE SQL data types.) We did have a problem with this in older versions of MySQL++, but we believe we fixed it completely in v3.0.2. No one has since proven data loss via this path. There is still a known problem [7] with the SQL DECIMAL type, which is somewhat related to the floating-point issue, but it’s apparently rarely encountered, which is why it hasn’t been fixed yet.
The best way to avoid problems with data translation
    is to always use the special MySQL++ data types defined in
    lib/sql_types.h corresponding to your
    SQL schema. These typedefs begin with sql_ and
    end with a lowercase version of the standard SQL type name,
    with spaces replaced by underscores. There are variants
    ending in _null that wrap these base types
    so they’re compatible with
    SQL null. For instance, the SQL type TINYINT
    UNSIGNED NOT NULL is represented in MySQL++ by
    mysqlpp::sql_tinyint_unsigned. If you drop
    the NOT NULL part, the corresponding C++ type is
    mysqlpp::sql_tinyint_unsigned_null.
MySQL++ doesn’t force you to use these typedefs. It
    tries to be flexible with regard to data conversions,
    so you could probably use int anywhere you
    use mysqlpp::sql_tinyint_unsigned,
    for example. That said, the MySQL++ typedefs give several
    advantages:
Space efficiency: the MySQL++ types are no larger than necessary to hold the MySQL data.
Portability: if your program has to run on multiple different system types (even just 32- and 64-bit versions of the same operating system and processor type) using the MySQL++ typedefs insulates your code from platform changes.
Clarity: using C++ types named similarly to the SQL types reduces the risk of confusion when working with code in both languages at the same time.
Compatibility: using the MySQL++ types ensures that data conversions between SQL and C++ forms are compatible. Naïve use of plain old C++ types can result in data truncation, TypeLookupFailed exceptions, and worse.
Type compatibility is important not just at the time
      you write your program, it also helps forward compatibility:
      we occasionally change the definitions of the MySQL++
      typedefs to reduce the differences between the C++
      and SQL type systems. We’ll be fixing the DECIMAL issue
      brought up above this way, for instance; if your program
      uses sql_decimal instead of the
      current underlying type, double, your program
      will pick up this improvement automatically with just a
      recompile.
Most of these typedefs use standard C++ data types, but
    a few are aliases for a MySQL++ specific type. For instance,
    the SQL type DATETIME is mirrored in
    MySQL++ by mysqlpp::DateTime. For
    consistency, sql_types.h includes a
    typedef alias for DateTime called
    mysqlpp::sql_datetime.
MySQL++ doesn’t have typedefs for the most exotic data types, like those for the geospatial types. Patches to correct this will be thoughtfully considered.
Both C++ and SQL have things in them called NULL, but they differ in several ways. Consequently, MySQL++ has to provide special support for this, rather than just wrap native C++ facilities as it can with most data type issues.
The primary distinction is one of type. In SQL, “NULL” is a type modifier, which affects whether you can legally store a null value in that column. There’s simply nothing like it in C++.
To emulate SQL NULL, MySQL++ provides the Null template to allow the creation of distinct “nullable” versions of existing C++ types. So for example, if you have a TINYINT UNSIGNED column that can have nulls, the proper declaration for MySQL++ would be:
mysqlpp::Null<mysqlpp::sql_tinyint_unsigned> myfield;
As of MySQL++ 3.1, we also provide shorter aliases for such types:
mysqlpp::sql_tinyint_unsigned_null myfield;
These types are declared in
      lib/sql_types.h. You might want to scan
      through that to see what all is available.
Template instantiations are first-class types in the C++ language, so there’s no possible confusion between this feature of MySQL++ and C++’s native NULL concept.
There’s a secondary distinction between SQL null and
      anything available in the standard C++ type system: SQL null
      is a distinct value, equal to nothing else. We can’t
      use C++’s NULL for this because it
      is ambiguous, being equal to 0 in integer context. MySQL++
      provides the global null object, which you
      can assign to a Null template instance
      to make it equal to SQL null:
myfield = mysqlpp::null;
If you insert a MySQL++ field holding a SQL null into a C++ IOstream, you get “(NULL)”, something fairly unlikely to be in a normal output string, thus reasonably preserving the uniqueness of the SQL null value.
MySQL++ also tries to enforce the uniqueness of the
      SQL null value at compile time in assignments and data
      conversions. If you try to store a SQL null in a field type
      that isn’t wrapped by Null
      or try to assign a Null-wrapped
      field value to a variable of the inner non-wrapped type,
      the compiler will emit some ugly error message, yelling about
      CannotConvertNullToAnyOtherDataType.  (The exact
      message is compiler-dependent.)
If you don’t like these behaviors, you can change
      them by passing a different value for the second parameter
      to template Null. By default, this
      parameter is NullIsNull,
      meaning that we should enforce the uniqueness of SQL
      null. To relax the distinctions, you can instantiate the
      Null template with a different behavior
      type: NullIsZero or NullIsBlank. Consider this code:
mysqlpp::Null<unsigned char, mysqlpp::NullIsZero> myfield(mysqlpp::null); cout << myfield << endl; cout << int(myfield) << endl;
This will print “0” twice. If you had used the
      default for the second Null template
      parameter, the first output statement would have printed
      “(NULL)”, and the second wouldn’t even
      compile.
MySQL++ has two classes that work like
    std::string to some degree: String and SQLTypeAdapter. These classes exist to provide functionality
    that std::string doesn’t provide, but
    they are neither derivatives of nor complete supersets of
    std::string.  As a result, end-user code
    generally doesn’t deal with these classes directly, because
    std::string is a better general-purpose
    string type. In fact, MySQL++ itself uses
    std::string most of the time, too. But, the
    places these specialized stringish types do get used are so
    important to the way MySQL++ works that it’s well worth taking
    the time to understand them.
The simpler of the two is
      SQLTypeAdapter, or
      STA for short.[8]
As its name suggests, its only purpose is to adapt other data types to be used with SQL. It has a whole bunch of conversion constructors, one for all data types we expect to be used with MySQL++ for values in queries. SQL queries are strings, so constructors that take stringish types just make a copy of that string, and all the others “stringize” the value in the format needed by SQL.[9] The conversion constructors preserve type information, so this stringization process doesn’t throw away any essential information.
STA is used anywhere MySQL++ needs to
      be able to accept any of several data types for use in a SQL
      query. Major users are Query’s
      template query mechanism and the Query
      stream quoting and escaping mechanism. You care about
      STA because any time you pass a data value
      to MySQL++ to be used in building a SQL query, it goes through
      STA. STA is one of
      the key pieces in MySQL++ that makes it easy to generate
      syntactically-correct SQL queries.
If MySQL++ can be said to have its own generic string type,
      it’s String, but it’s not
      really functional enough for general use. It’s possible that
      in future versions of MySQL++ we’ll expand its interface to
      include everything std::string does, so
      that’s why it’s called that.[10]
The key thing String provides over
      std::string is conversion of strings in SQL
      value formats to their plain old C++ data types. For example, if you
      initialize it with the string “2007-11-19”, you can
      assign the String to a Date, not because
      Date knows how to initialize itself from
      String, but the reverse:
      String has a bunch of implicit conversion
      operators defined for it, so you can use it in any type context
      that makes sense in your application.
Because Row::operator[] returns
      String, you can say things like
      this:
int x = row["x"];
In a very real sense, String is the
      inverse of STA:
      String converts SQL value strings to C++
      data types, and STA converts C++ data types
      to SQL value strings.[11]
String has two main uses.
By far the most common use is as the field value type of
      Row, as exemplified above. It’s not
      just the return type of Row::operator[],
      though: it’s actually the value type used within
      Row’s internal array. As a result,
      any time MySQL++ pulls data from the database, it goes through
      String when converting it from the string
      form used in SQL result sets to the C++ data type you actually
      want the data in. It’s the core of the structure population
      mechanism in the SSQLS feature, for
      example.
Because String is the last pristine
      form of data in a result set before it gets out of MySQL++’s
      internals where end-user code can see it, MySQL++’s
      sql_blob and related typedefs are
      aliases for String. Using anything else
      would require copies; while the whole “networked database
      server” thing means most of MySQL++ can be quite inefficient
      and still not affect benchmark results meaningfully, BLOBs tend to
      be big, so making unnecessary copies can really make a difference.
      Which brings us to...
To avoid unnecessary buffer copies, both
      STA and String
      are implemented in terms of a reference-counted copy-on-write
      buffer scheme. Both classes share the same underlying mechanism,
      and so are interoperable. This means that if you construct
      one of these objects from another, it doesn’t actually
      copy the string data, it only copies a pointer to the data
      buffer, and increments its reference count. If the object
      has new data assigned to it or it’s otherwise modified,
      it decrements its reference count and creates its own copy of
      the buffer. This has a lot of practical import, such as the
      fact that even though Row::operator[]
      returns Strings by value, it’s
      still efficient.
Historically, there was no way to hold arbitrary-sized blocks of raw binary data in an SQL database. There was resistance to adding such a feature to SQL for a long time because it’s better, where possible, to decompose blocks of raw binary data into a series of numbers and text strings that can be stored in the database. This lets you query, address and manipulate elements of the data block individually.
A classic SQL newbie mistake is trying to treat the database server as a file system. Some embedded platforms use a database engine as a file system, but MySQL doesn’t typically live in that world. When your platform already has a perfectly good file system, you should use it for big, nondecomposable blocks of binary data in most cases.
A common example people use when discussing this is images in database-backed web applications. If you store the image in the database, you have to write code to retrieve the image from the database and send it to the client; there’s more overhead, and less efficient use of the system’s I/O caching system. If you store the image in the filesystem, all you have to do is point the web server to the directory where the images live, and put a URL for that image in your generated HTML. Because you’re giving the web server a direct path to a file on disk, operation is far more efficient. Web servers are very good at slurping whole files off of disk and sending them out to the network, and operating systems are very good at caching file accesses. Plus, you avoid the overhead of pushing the data through the high-level language your web app is written in, which is typically an interpreted language, not C++. Some people still hold out on this, claiming that database engines have superior security features, but I call bunk on that, too. Operating systems and web servers are capable of building access control systems every bit as granular and secure as a database system.
Occasionally you really do need to store a nondecomposable block of binary data in the database. For such cases, modern SQL database servers support BLOB data types, for Binary Large OBject. This is often just called binary data, though of course all data in a modern computer is binary at some level.
The tricky part about dealing with binary data in MySQL++ is to ensure that you don’t ever treat the data as a C string, which is really easy to do accidentally. C strings treat zero bytes as special end-of-string characters, but they’re not special at all in binary data. We’ve made a lot of improvements to the way MySQL++ handles string data to avoid this problem, but it’s still possible to bypass these features, wrecking your BLOBs. These examples demonstrate correct techniques.
Above, I opined that it’s usually incorrect to store image data in a database, particularly with web apps, of which CGI is a primitive form. Still, it makes a nice, simple example.
Instead of a single example program, we have here a matched pair. The first example takes the name of a JPEG file on the command line along with all the other common example program parameters, loads that file into memory, and stores it in a BLOB column in the database.
This example also demonstrates how to retrieve the value assigned to an auto-increment column in the previous insertion. This example uses that feature in the typical way, to create unique IDs for rows as they’re inserted.
Here is examples/load_jpeg.cpp:
#include "cmdline.h"
#include "images.h"
#include "printdata.h"
#include <fstream>
using namespace std;
using namespace mysqlpp;
// This is just an implementation detail for the example.  Skip down to
// main() for the concept this example is trying to demonstrate.  You
// can simply assume that, given a BLOB containing a valid JPEG, it
// returns true.
static bool
is_jpeg(const mysqlpp::sql_blob& img, const char** whynot)
{
    // See http://stackoverflow.com/questions/2253404/ for
    // justification for the various tests.
    const unsigned char* idp =
            reinterpret_cast<const unsigned char*>(img.data());
    if (img.size() < 125) {
        *whynot = "a valid JPEG must be at least 125 bytes";
    }
    else if ((idp[0] != 0xFF) || (idp[1] != 0xD8)) {
        *whynot = "file does not begin with JPEG sigil bytes";
    }
    else if ((memcmp(idp + 6, "JFIF", 4) != 0) &&
             (memcmp(idp + 6, "Exif", 4) != 0)) {
        *whynot = "file does not contain JPEG type word";
    }
    else {
        *whynot = 0;
        return true;
    }
    return false;
}
// Skip to main() before studying this.  This is a little too
// low-level to bother with on your first pass thru the code.
static bool
load_jpeg_file(const mysqlpp::examples::CommandLine& cmdline,
        images& img, string& img_name)
{
    if (cmdline.extra_args().size() == 0) {
        // Nothing for us to do here.  Caller will insert NULL BLOB.
        return true;
    }
    // Got a file's name on the command line, so open it.
    img_name = cmdline.extra_args()[0];
    ifstream img_file(img_name.c_str(), ios::binary);
    if (img_file) {
        // Slurp file contents into RAM with minimum copying.  (Idiom
        // explained here: http://stackoverflow.com/questions/116038/)
        //
        // By loading the file into a C++ string (stringstream::str())
        // and assigning that directly to a mysqlpp::sql_blob, we avoid
        // truncating the binary data at the first null character.
        img.data.data = static_cast<const stringstream*>(
                &(stringstream() << img_file.rdbuf()))->str();
        // Check JPEG data for sanity.
        const char* error;
        if (is_jpeg(img.data.data, &error)) {
            return true;
        }
        else {
            cerr << '"' << img_name << "\" isn't a JPEG: " <<
                    error << '!' << endl;
        }
    }
    cmdline.print_usage("[jpeg_file]");
    return false;
}
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        // Load the file named on the command line
        images img(mysqlpp::null, mysqlpp::null);
        string img_name("NULL");
        if (load_jpeg_file(cmdline, img, img_name)) {
            // Insert image data or SQL NULL into the images.data BLOB
            // column.  The key here is that we're holding the raw
            // binary data in a mysqlpp::sql_blob, which avoids data
            // conversion problems that can lead to treating BLOB data
            // as C strings, thus causing null-truncation.  The fact
            // that we're using SSQLS here is a side issue, simply
            // demonstrating that mysqlpp::Null<mysqlpp::sql_blob> is
            // now legal in SSQLS, as of MySQL++ 3.0.7.
            Query query = con.query();
            query.insert(img);
            SimpleResult res = query.execute();
            // Report successful insertion
            cout << "Inserted \"" << img_name <<
                    "\" into images table, " << img.data.data.size() <<
                    " bytes, ID " << res.insert_id() << endl;
        }
    }
    catch (const BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const BadConversion& er) {
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }
    return 0;
}
Notice that we used the escape manipulator when building the INSERT query above. This is because mysqlpp::sql_blob is just an alias for one of the special MySQL++ string types, which don’t do automatic quoting and escaping. They can’t, because MySQL++ also uses these data types to hold raw SQL query strings, which would break due to doubled quoting and/or escaping if it were automatic.
The other example in this pair is rather short,
      considering how much it does. It parses a CGI query string
      giving the image ID, uses that to retreive data loaded into
      the database by load_jpeg, and writes
      it out in the form a web server wants when processing a CGI
      call, all with adequate real-world error handling. This is
      examples/cgi_jpeg.cpp:
#include "cmdline.h"
#include "images.h"
#define CRLF            "\r\n"
#define CRLF2           "\r\n\r\n"
int
main(int argc, char* argv[])
{
    // Get database access parameters from command line if present, else
    // use hard-coded values for true CGI case.
    mysqlpp::examples::CommandLine cmdline(argc, argv, "root",
            "nunyabinness");
    if (!cmdline) {
        return 1;
    }
    // Parse CGI query string environment variable to get image ID
    unsigned int img_id = 0;
    char* cgi_query = getenv("QUERY_STRING");
    if (cgi_query) {
        if ((strlen(cgi_query) < 4) || memcmp(cgi_query, "id=", 3)) {
            std::cout << "Content-type: text/plain" << std::endl << std::endl;
            std::cout << "ERROR: Bad query string" << std::endl;
            return 1;
        }
        else {
            img_id = atoi(cgi_query + 3);
        }
    }
    else {
        std::cerr << "Put this program into a web server's cgi-bin "
                "directory, then" << std::endl;
        std::cerr << "invoke it with a URL like this:" << std::endl;
        std::cerr << std::endl;
        std::cerr << "    http://server.name.com/cgi-bin/cgi_jpeg?id=2" <<
                std::endl;
        std::cerr << std::endl;
        std::cerr << "This will retrieve the image with ID 2." << std::endl;
        std::cerr << std::endl;
        std::cerr << "You will probably have to change some of the #defines "
                "at the top of" << std::endl;
        std::cerr << "examples/cgi_jpeg.cpp to allow the lookup to work." <<
                std::endl;
        return 1;
    }
    // Retrieve image from DB by ID
    try {
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        mysqlpp::Query query = con.query();
        query << "SELECT * FROM images WHERE id = " << img_id;
        mysqlpp::StoreQueryResult res = query.store();
        if (res && res.num_rows()) {
            images img = res[0];
            if (img.data.is_null) {
                std::cout << "Content-type: text/plain" << CRLF2;
                std::cout << "No image content!" << CRLF;
            }
            else {
                std::cout << "X-Image-Id: " << img_id << CRLF; // for debugging
                std::cout << "Content-type: image/jpeg" << CRLF;
                std::cout << "Content-length: " <<
                        img.data.data.length() << CRLF2;
                std::cout << img.data;
            }
        }
        else {
            std::cout << "Content-type: text/plain" << CRLF2;
            std::cout << "ERROR: No image with ID " << img_id << CRLF;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        std::cout << "Content-type: text/plain" << CRLF2;
        std::cout << "QUERY ERROR: " << er.what() << CRLF;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cout << "Content-type: text/plain" << CRLF2;
        std::cout << "GENERAL ERROR: " << er.what() << CRLF;
        return 1;
    }
    return 0;
}
While you can run it by hand, it’s
      best to install this in a web server’s
      CGI program directory, then call it with a URL like
      http://my.server.com/cgi-bin/cgi_jpeg?id=1.
      That retrieves the JPEG with ID 1 from the database and
      returns it to the web server, which will send it on to the
      browser.
We’ve included an image with MySQL++
      that you can use with this example pair,
      examples/logo.jpg.
The Transaction class makes it
    easier to use SQL transactions in an exception-safe manner. Normally
    you create the Transaction object on the
    stack before you issue the queries in your transaction set. Then,
    when all the queries in the transaction set have been issued, you
    call Transaction::commit(), which commits the
    transaction set. If the Transaction object
    goes out of scope before you call commit(), the
    transaction set is rolled back. This ensures that if some code
    throws an exception after the transaction is started but before it
    is committed, the transaction isn’t left unresolved.
examples/transaction.cpp illustrates
    this:
#include "cmdline.h"
#include "printdata.h"
#include "stock.h"
#include <iostream>
#include <cstdio>
using namespace std;
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        // Show initial state
        mysqlpp::Query query = con.query();
        cout << "Initial state of stock table:" << endl;
        print_stock_table(query);
        // Insert a few rows in a single transaction set
        {
            // Use a higher level of transaction isolation than MySQL
            // offers by default.  This trades some speed for more
            // predictable behavior.  We've set it to affect all
            // transactions started through this DB server connection,
            // so it affects the next block, too, even if we don't
            // commit this one.
            mysqlpp::Transaction trans(con,
                    mysqlpp::Transaction::serializable,
                    mysqlpp::Transaction::session);
            stock row("Sauerkraut", 42, 1.2, 0.75,
                    mysqlpp::sql_date("2006-03-06"), mysqlpp::null);
            query.insert(row);
            query.execute();
            cout << "\nRow inserted, but not committed." << endl;
            cout << "Verify this with another program (e.g. simple1), "
                    "then hit Enter." << endl;
            getchar();
            cout << "\nCommitting transaction gives us:" << endl;
            trans.commit();
            print_stock_table(query);
        }
            
        // Now let's test auto-rollback
        {
            // Start a new transaction, keeping the same isolation level
            // we set above, since it was set to affect the session.
            mysqlpp::Transaction trans(con);
            cout << "\nNow adding catsup to the database..." << endl;
            stock row("Catsup", 3, 3.9, 2.99,
                    mysqlpp::sql_date("2006-03-06"), mysqlpp::null);
            query.insert(row);
            query.execute();
        }
        cout << "\nNo, yuck! We don't like catsup. Rolling it back:" <<
                endl;
        print_stock_table(query);
            
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {  
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }
    return 0;
}
One of the downsides of transactions is that the locking it requires in the database server is prone to deadlocks. The classic case where this happens is when two programs both want access to the same two rows within a single transaction each, but they modify them in opposite orders. If the timing is such that the programs interleave their lock acquisitions, the two come to an impasse: neither can get access to the other row they want to modify until the other program commits its transaction and thus release the row locks, but neither can finish the transaction because they’re waiting on row locks the database server is holding on behalf of the other program.
The MySQL server is smart enough to detect this condition, but the best it can do is abort the second transaction. This breaks the impasse, allowing the first program to complete its transaction.
The second program now has to deal with the fact that its
    transaction just got aborted. There’s a subtlety in detecting
    this situation when using MySQL++. By default, MySQL++ signals
    errors like these with exceptions. In the exception handler, you
    might expect to get ER_LOCK_DEADLOCK from
    Query::errnum() (or
    Connection::errnum(), same thing), but what
    you’ll almost certainly get instead is 0, meaning “no
    error.” Why? It’s because you’re probably using a
    Transaction object to get automatic
    roll-backs in the face of exceptions. In this case, the roll-back
    happens before your exception handler is called by issuing a
    ROLLBACK query to the database server. Thus,
    Query::errnum() returns the error code
    associated with this roll-back query, not the deadlocked transaction
    that caused the exception.
To avoid this problem, a few of the exception objects as of
    MySQL++ v3.0 include this last error number in the exception object
    itself. It’s populated at the point of the exception, so it
    can differ from the value you would get from
    Query::errnum() later on when the exception
    handler runs.
The example examples/deadlock.cpp
    demonstrates the problem:
#include "cmdline.h"
#include <mysql++.h>
#include <mysqld_error.h>
#include <iostream>
using namespace std;
// Bring in global holding the value given to the -m switch
extern int run_mode;
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    // Check that the mode parameter was also given and it makes sense
    const int run_mode = cmdline.run_mode();
    if ((run_mode != 1) && (run_mode != 2)) {
        cerr << argv[0] << " must be run with -m1 or -m2 as one of "
                "its command-line arguments." << endl;
        return 1;
    }
    mysqlpp::Connection con;
    try {
        // Establish the connection to the database server
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        // Start a transaction set.  Transactions create mutex locks on
        // modified rows, so if two programs both touch the same pair of
        // rows but in opposite orders at the wrong time, one of the two
        // programs will deadlock.  The MySQL server knows how to detect
        // this situation, and its error return causes MySQL++ to throw
        // a BadQuery exception.  The point of this example is that if
        // you want to detect this problem, you would check the value of
        // BadQuery::errnum(), not Connection::errnum(), because the
        // transaction rollback process executes a query which succeeds,
        // setting the MySQL C API's "last error number" value to 0.
        // The exception object carries its own copy of the error number
        // at the point the exception was thrown for this very reason.
        mysqlpp::Query query = con.query();
        mysqlpp::Transaction trans(con);
        // Build and run the queries, with the order depending on the -m
        // flag, so that a second copy of the program will deadlock if
        // run while the first is waiting for Enter.
        char dummy[100];
        for (int i = 0; i < 2; ++i) {
            int lock = run_mode + (run_mode == 1 ? i : -i);
            cout << "Trying lock " << lock << "..." << endl;
            query << "select * from deadlock_test" << lock << 
                    " where x = " << lock << " for update";
            query.store();
            cout << "Acquired lock " << lock << ".  Press Enter to ";
            cout << (i == 0 ? "try next lock" : "exit");
            cout << ": " << flush;
            cin.getline(dummy, sizeof(dummy));
        }
    }
    catch (mysqlpp::BadQuery e) {
        if (e.errnum() == ER_LOCK_DEADLOCK) {
            cerr << "Transaction deadlock detected!" << endl;
            cerr << "Connection::errnum = " << con.errnum() <<
                    ", BadQuery::errnum = " << e.errnum() << endl;
        }
        else {
            cerr << "Unexpected query error: " << e.what() << endl;
        }
        return 1;
    }
    catch (mysqlpp::Exception e) {
        cerr << "General error: " << e.what() << endl;      
        return 1;
    }
    return 0;
}
This example works a little differently than the others. You run one copy of the example, then when it pauses waiting for you to press Enter, you run another copy. Then, depending on which one you press Enter in, one of the two will abort with the deadlock exception. You can see from the error message you get that it matters which method you call to get the error number. What you do about it is up to you as it depends on your program’s design and system architecture.
There are three major ways to execute a query in MySQL++:
    Query::execute(),
    Query::store(), and
    Query::use(). Which should you use, and
    why?
execute() is for queries that do not
    return data per se. For instance,
    CREATE INDEX. You do get back some information
    from the MySQL server, which execute()
    returns to its caller in a SimpleResult object. In addition to the obvious — a
    flag stating whether the query succeeded or not — this object
    also contains things like the number of rows that the query
    affected. If you only need the success status, it’s a little
    more efficient to call Query::exec()
    instead, as it simply returns bool.
If your query does pull data from the database, the simplest
    option is store(). (All of the examples up
    to this point have used this method.)  This returns a StoreQueryResult object, which contains the
    entire result set. It’s especially convenient because
    StoreQueryResult derives from
    std::vector<mysqlpp::Row>, so it opens
    the whole panoply of STL operations for accessing the rows in the
    result set. Access rows randomly with subscript notation, iterate
    forwards and backwards over the result set, run STL algorithms on
    the set...it all works naturally.
If you like the idea of storing your results in an STL
    container but don’t want to use
    std::vector, you can call
    Query::storein() instead. It lets you store
    the results in any standard STL container (yes, both sequential and
    set-associative types) instead of using
    StoreQueryResult. You do miss out on some of
    the additional database information held by
    StoreQueryResult’s other base class,
    ResultBase, however.
store*() queries are convenient, but
    the cost of keeping the entire result set in main memory can
    sometimes be too high. It can be surprisingly costly, in fact. A
    MySQL database server stores data compactly on disk, but it returns
    query data to the client in a textual form. This results in a kind
    of data bloat that affects numeric and BLOB types the most. MySQL++
    and the underlying C API library also have their own memory
    overheads in addition to this. So, if you happen to know that the
    database server stores every record of a particular table in 1 KB,
    pulling a million records from that table could easily take several
    GB of memory with a store() query,
    depending on what’s actually stored in that table.
For these large result sets, the superior option is a
    use() query. This returns a UseQueryResult object, which is similar to
    StoreQueryResult, but without all of the
    random-access features. This is because a “use” query
    tells the database server to send the results back one row at a
    time, to be processed linearly. It’s analogous to a C++
    stream’s input iterator, as opposed to a random-access
    iterator that a container like vector offers. By accepting this
    limitation, you can process arbitrarily large result sets. This
    technique is demonstrated in
    examples/simple3.cpp:
#include "cmdline.h"
#include "printdata.h"
#include <mysql++.h>
#include <iostream>
#include <iomanip>
using namespace std;
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    // Connect to the sample database.
    mysqlpp::Connection conn(false);
    if (conn.connect(mysqlpp::examples::db_name, cmdline.server(),
            cmdline.user(), cmdline.pass())) {
        // Ask for all rows from the sample stock table and display
        // them.  Unlike simple2 example, we retreive each row one at
        // a time instead of storing the entire result set in memory
        // and then iterating over it.
        mysqlpp::Query query = conn.query("select * from stock");
        if (mysqlpp::UseQueryResult res = query.use()) {
            // Display header
            cout.setf(ios::left);
            cout << setw(31) << "Item" <<
                    setw(10) << "Num" <<
                    setw(10) << "Weight" <<
                    setw(10) << "Price" <<
                    "Date" << endl << endl;
            // Get each row in result set, and print its contents
            while (mysqlpp::Row row = res.fetch_row()) {
                cout << setw(30) << row["item"] << ' ' <<
                        setw(9) << row["num"] << ' ' <<
                        setw(9) << row["weight"] << ' ' <<
                        setw(9) << row["price"] << ' ' <<
                        setw(9) << row["sdate"] <<
                        endl;
            }
            // Check for error: can't distinguish "end of results" and
            // error cases in return from fetch_row() otherwise.
            if (conn.errnum()) {
                cerr << "Error received in fetching a row: " <<
                        conn.error() << endl;
                return 1;
            }
            return 0;
        }
        else {
            cerr << "Failed to get stock item: " << query.error() << endl;
            return 1;
        }
    }
    else {
        cerr << "DB connection failed: " << conn.error() << endl;
        return 1;
    }
}
This example does the same thing as
    simple2, only with a “use” query
    instead of a “store” query.
Valuable as use() queries are, they
    should not be the first resort in solving problems of excessive
    memory use. It’s better if you can find a way to simply not
    pull as much data from the database in the first place. Maybe
    you’re saying SELECT * even though you
    don’t immedidately need all the columns from the table. Or,
    maybe you’re filtering the result set with C++ code after you
    get it from the database server. If you can do that filtering with a
    more restrictive WHERE clause on the
    SELECT, it’ll not only save memory,
    it’ll save bandwidth between the database server and client,
    and can even save CPU time. If the filtering criteria can’t be
    expressed in a WHERE clause, however, read on to
    the next section.
Sometimes you must pull more data from the database server
    than you actually need and filter it in memory. SQL’s
    WHERE clause is powerful, but not as powerful as
    C++. Instead of storing the full result set and then picking over it
    to find the rows you want to keep, use
    Query::store_if(). This is
    examples/store_if.cpp:
#include "cmdline.h"
#include "printdata.h"
#include "stock.h"
#include <mysql++.h>
#include <iostream>
#include <math.h>
// Define a functor for testing primality.
struct is_prime
{
    bool operator()(const stock& s)
    {
        if ((s.num == 2) || (s.num == 3)) {
            return true;    // 2 and 3 are trivial cases
        }
        else if ((s.num < 2) || ((s.num % 2) == 0)) {
            return false;   // can't be prime if < 2 or even
        }
        else {
            // The only possibility left is that it's divisible by an
            // odd number that's less than or equal to its square root.
            for (int i = 3; i <= sqrt(double(s.num)); i += 2) {
                if ((s.num % i) == 0) {
                    return false;
                }
            }
            return true;
        }
    }
};
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        // Collect the stock items with prime quantities
        std::vector<stock> results;
        mysqlpp::Query query = con.query();
        query.store_if(results, stock(), is_prime());
        // Show the results
        print_stock_header(results.size());
        std::vector<stock>::const_iterator it;
        for (it = results.begin(); it != results.end(); ++it) {
            print_stock_row(it->item.c_str(), it->num, it->weight,
                    it->price, it->sDate);
        }
    }
    catch (const mysqlpp::BadQuery& e) {
        // Something went wrong with the SQL query.
        std::cerr << "Query failed: " << e.what() << std::endl;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cerr << "Error: " << er.what() << std::endl;
        return 1;
    }
    return 0;
}
I doubt anyone really needs to select rows from a table that
    have a prime number in a given field. This example is meant to be
    just barely more complex than SQL can manage, to avoid obscuring the
    point. That point being, the
    Query::store_if() call here gives you a
    container full of results meeting a criterion that you probably
    can’t express in SQL. You will no doubt have much more useful
    criteria in your own programs.
If you need a more complex query than the one
    store_if() knows how to build when given an
    SSQLS examplar, there are two overloads that let you use your own
    query string. One overload takes the query string directly, and the
    other uses the query string built with
    Query’s stream interface.
SQL is more than just a database query language. Modern
    database engines can actually do some calculations on the data on
    the server side. But, this isn’t always the best way to get
    something done. When you need to mix code and a query,
    MySQL++’s Query::for_each() facility
    might be just what you need. This is
    examples/for_each.cpp:
#include "cmdline.h"
#include "printdata.h"
#include "stock.h"
#include <mysql++.h>
#include <iostream>
#include <math.h>
// Define a functor to collect statistics about the stock table
class gather_stock_stats
{
public:
    gather_stock_stats() :
    items_(0),
    weight_(0),
    cost_(0)
    {
    }
    void operator()(const stock& s)
    {
        items_  += s.num;
        weight_ += (s.num * s.weight);
        cost_   += (s.num * s.price.data);
    }
    
private:
    mysqlpp::sql_bigint items_;
    mysqlpp::sql_double weight_, cost_;
    friend std::ostream& operator<<(std::ostream& os,
            const gather_stock_stats& ss);
};
// Dump the contents of gather_stock_stats to a stream in human-readable
// form.
std::ostream&
operator<<(std::ostream& os, const gather_stock_stats& ss)
{
    os << ss.items_ << " items " <<
            "weighing " << ss.weight_ << " stone and " <<
            "costing " << ss.cost_ << " cowrie shells";
    return os;
}
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        // Gather and display the stats for the entire stock table
        mysqlpp::Query query = con.query();
        std::cout << "There are " << query.for_each(stock(),
                gather_stock_stats()) << '.' << std::endl;
    }
    catch (const mysqlpp::BadQuery& e) {
        // Something went wrong with the SQL query.
        std::cerr << "Query failed: " << e.what() << std::endl;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cerr << "Error: " << er.what() << std::endl;
        return 1;
    }
    return 0;
}
You only need to read the main() function
    to get a good idea of what the program does. The key line of code
    passes an SSQLS examplar and a functor to
    Query::for_each().
    for_each() uses the SSQLS instance to build
    a select * from TABLE query,
    stock in this case. It runs that
    query internally, calling gather_stock_stats
    on each row. This is a pretty contrived example; you could actually
    do this in SQL, but we’re trying to prevent the complexity of
    the code from getting in the way of the demonstration here.
Just as with store_if(), described
    above, there are two other overloads for
    for_each() that let you use your own query
    string.
MySQL has a large number of options that control how it makes
    the connection to the database server, and how that connection
    behaves. The defaults are sufficient for most programs, so only one
    of the MySQL++ example programs make any connection option changes.
    Here is examples/multiquery.cpp:
#include "cmdline.h"
#include "printdata.h"
#include <mysql++.h>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <vector>
using namespace std;
using namespace mysqlpp;
typedef vector<size_t> IntVectorType;
static void
print_header(IntVectorType& widths, StoreQueryResult& res)
{
    cout << "  |" << setfill(' ');
    for (size_t i = 0; i < res.field_names()->size(); i++) {
        cout << " " << setw(widths.at(i)) << res.field_name(int(i)) << " |";
    }
    cout << endl;
}
static void
print_row(IntVectorType& widths, Row& row)
{
    cout << "  |" << setfill(' ');
    for (size_t i = 0; i < row.size(); ++i) {
        cout << " " << setw(widths.at(i)) << row[int(i)] << " |";
    }
    cout << endl;
}
static void
print_row_separator(IntVectorType& widths)
{
    cout << "  +" << setfill('-');
    for (size_t i = 0; i < widths.size(); i++) {
        cout << "-" << setw(widths.at(i)) << '-' << "-+";
    }
    cout << endl;
}
static void
print_result(StoreQueryResult& res, int index)
{
    // Show how many rows are in result, if any
    StoreQueryResult::size_type num_results = res.size();
    if (res && (num_results > 0)) {
        cout << "Result set " << index << " has " << num_results <<
                " row" << (num_results == 1 ? "" : "s") << ':' << endl;
    }
    else {
        cout << "Result set " << index << " is empty." << endl;
        return;
    }
    // Figure out the widths of the result set's columns
    IntVectorType widths;
    size_t size = res.num_fields();
    for (size_t i = 0; i < size; i++) {
        widths.push_back(max(
                res.field(i).max_length(),
                res.field_name(i).size()));
    }
    // Print result set header
    print_row_separator(widths);
    print_header(widths, res);
    print_row_separator(widths);
    // Display the result set contents
    for (StoreQueryResult::size_type i = 0; i < num_results; ++i) {
        print_row(widths, res[i]);
    }
    // Print result set footer
    print_row_separator(widths);
}
static void
print_multiple_results(Query& query)
{
    // Execute query and print all result sets
    StoreQueryResult res = query.store();
    print_result(res, 0);
    for (int i = 1; query.more_results(); ++i) {
        res = query.store_next();
        print_result(res, i);
    }
}
int
main(int argc, char *argv[])
{
    // Get connection parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    try {
        // Enable multi-queries.  Notice that you almost always set
        // MySQL++ connection options before establishing the server
        // connection, and options are always set using this one
        // interface.  If you're familiar with the underlying C API,
        // you know that there is poor consistency on these matters;
        // MySQL++ abstracts these differences away.
        Connection con;
        con.set_option(new MultiStatementsOption(true));
        // Connect to the database
        if (!con.connect(mysqlpp::examples::db_name, cmdline.server(),
                cmdline.user(), cmdline.pass())) {
            return 1;
        }
        // Set up query with multiple queries.
        Query query = con.query();
        query << "DROP TABLE IF EXISTS test_table; " <<
                "CREATE TABLE test_table(id INT); " <<
                "INSERT INTO test_table VALUES(10); " <<
                "UPDATE test_table SET id=20 WHERE id=10; " <<
                "SELECT * FROM test_table; " <<
                "DROP TABLE test_table";
        cout << "Multi-query: " << endl << query << endl;
        // Execute statement and display all result sets.
        print_multiple_results(query);
#if MYSQL_VERSION_ID >= 50000
        // If it's MySQL v5.0 or higher, also test stored procedures, which
        // return their results the same way multi-queries do.
        query << "DROP PROCEDURE IF EXISTS get_stock; " <<
                "CREATE PROCEDURE get_stock" <<
                "( i_item varchar(20) ) " <<
                "BEGIN " <<
                "SET i_item = concat('%', i_item, '%'); " <<
                "SELECT * FROM stock WHERE lower(item) like lower(i_item); " <<
                "END;";
        cout << "Stored procedure query: " << endl << query << endl;
        // Create the stored procedure.
        print_multiple_results(query);
        // Call the stored procedure and display its results.
        query << "CALL get_stock('relish')";
        cout << "Query: " << query << endl;
        print_multiple_results(query);
#endif
        return 0;
    }
    catch (const BadOption& err) {
        cerr << err.what() << endl;
        cerr << "This example requires MySQL 4.1.1 or later." << endl;
        return 1;
    }
    catch (const ConnectionFailed& err) {
        cerr << "Failed to connect to database server: " <<
                err.what() << endl;
        return 1;
    }
    catch (const Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return 1;
    }
}
This is a fairly complex example demonstrating the multi-query
    and stored procedure features in newer versions of MySQL. Because
    these are new features, and they change the communication between
    the client and server, you have to enable these features in a
    connection option. The key line is right up at the top of
    main(), where it creates a MultiStatementsOption object and passes it
    to Connection::set_option(). That method
    will take a pointer to any derivative of Option: you just create such an object on the heap and pass
    it in, which gives Connection the data values
    it needs to set the option. You don’t need to worry about
    releasing the memory used by the Option
    objects; it’s done automatically.
The only tricky thing about setting options is that only a few
    of them can be set after the connection is up. Most need to be set
    just as shown in the example above: create an unconnected
    Connection object, set your connection
    options, and only then establish the connection. The option setting
    mechanism takes care of applying the options at the correct time in
    the connection establishment sequence.
If you’re familiar with setting connection options in the MySQL C API, you’ll have to get your head around the fact that MySQL++’s connection option mechanism is a much simpler, higher-level design that doesn’t resemble the C API in any way. The C API has something like half a dozen different mechanisms for setting options that control the connection. The flexibility of the C++ type system allows us to wrap all of these up into a single high-level mechanism while actually getting greater type safety than the C API allows.
By default, current MySQL servers have an 8 hour idle timeout on connections. This is not a problem if your program never has to run for more than 8 hours or reliably queries the database more often than that. And, it’s a good thing for the database server, because even an idle connection takes up server resources.
Many programs must run continually, however, and may experience long idle periods, such as nights and weekends when no one is around to make the program issue database queries. It’s therefore common for people writing such programs to get a bug report from the field complaining that the program died overnight or over a long weekend, usually with some error message about the database server going away. They then check the DB server, find that it’s still running and never did restart and scratch their heads wondering what happened. What happened is that the server’s connection idle timeout expired, so it closed the connection to the client.
You cannot detect this condition by calling
    Connection::connected(). When
    that returns true, it just means
    that either the connect-on-create constructor or the
    connect() call succeeded and that we
    haven’t observed the connection to be down since then.
    When the database server closes an idle connection, you won’t
    know it until after you try to issue a query. This is simply due
    to the nature of network programming.
One way around this problem is to configure MySQL to have a longer idle timeout. This timeout is in seconds, so the default of 8 hours is 28,800 seconds. You would want to figure out the longest possible time that your program could be left idle, then pick a value somewhat longer than that. For instance, you might decide that the longest reasonable idle time is a long 4-day weekend — 345,600 seconds — which you could round up to 350,000 or 400,000 to allow for a little bit of additional idle time on either end of that period.
Another way around this, on a per-connection basis from the client side, would be to set the ReconnectOption connection option. This will cause MySQL++ to reconnect to the server automatically if it drops the connection. Beware that unless you’re using MySQL 5.1.6 or higher, you have to set this only after the connection is established, or it won’t take effect. This means there’s a potential race condition: it’s possible the connection could drop shortly enough after being established that you don’t have time to apply the option, so it won’t come back up automatically. MySQL 5.1.6+ fixes this by allowing this option to be set before the connection is established.
A completely different way to tackle this, if your program
    doesn’t block forever waiting on I/O while idle, is to
    periodically call Connection::ping().
    [12]
    This sends the smallest possible amount of data to the
    database server, which will reset its idle timer and cause
    it to respond, so ping() returns
    true. If it returns false
    instead, you know you need to reconnect to the server. Periodic
    pinging is easiest to do if your program uses asynchronous I/O,
    threads, or some kind of event
    loop to ensure that you can call something periodically even
    while the rest of the program has nothing to do.
An interesting variant on this strategy is to ping the server before each query, or, better, before each group of queries within a larger operation. It has an advantage over pinging during idle time in that the client is about to use far more server resources to handle the query than it will take to handle the ping, so the ping time gets lost in the overhead. On the other hand, if the client issues queries frequently when not idle, it can result in a lot more pings than would happen if you just pinged every N hours while idle.
Finally, some programmers prefer to wrap the querying mechanism in an error handler that catches the “server has gone away” error and tries to reestablish the connection and reissue the query. This adds some complexity, but it makes your program more robust without taking up unnecessary resources. If you did this, you could even change the server to drop idle connections more often, thus tying up fewer TCP/IP stack resources.
An important limitation of the MySQL C API library —
    which MySQL++ is built atop, so it shares this limitation —
    is that you can only have one query in progress on each connection
    to the database server. If you try to issue a second query while
    one is still in progress, you get an obscure error message about
    “Commands out of sync” from the underlying C API
    library. (You normally get this message in a MySQL++ exception
    unless you have exceptions disabled, in which case you get a
    failure code and Connection::error()
    returns this message.)
There are lots of ways to run into this limitation:
The easiest way is to try to use a single Connection object in a multithreaded program, with more than one thread attempting to use it to issue queries. Unless you put in a lot of work to synchronize access, this is almost guaranteed to fail at some point, giving the dread “Commands out of sync” error.
You might then think to give each thread that issues
        queries its own Connection object.
        You can still run into trouble if you pass the data you get
        from queries around to other threads. What can happen is
        that one of these child objects indirectly calls back to the
        Connection at a time where it’s
        involved with another query. This is properly covered
        elsewhere, in Section 7.4, “Sharing MySQL++ Data Structures”.)
One way to run into this problem without using threads is with “use” queries, discussed above. If you don’t consume all rows from a query before you issue another on that connection, you are effectively trying to have multiple concurrent queries on a single connection. Here’s a recipie for this particular disaster:
UseQueryResult r1 = query.use("select garbage from plink where foobie='tamagotchi'");
UseQueryResult r2 = query.use("select blah from bonk where bletch='smurf'");The second use() call fails because
        the first result set hasn’t been consumed yet.
Still another way to run into this limitation
        is if you use MySQL’s multi-query feature. This
        lets you give multiple queries in a single call,
        separated by semicolons, and get back the results for
        each query separately. If you issue three queries using
        Query::store(), you only get
        back the first query’s results with that call, and
        then have to call store_next()
        to get the subsequent query results.  MySQL++ provides
        Query::more_results() so
        you know whether you’re done, or need to call
        store_next() again.  Until you reach
        the last result set, you can’t issue another query on
        that connection.
Finally, there’s a way to run into this
        that surprises almost everyone sooner or later: stored
        procedures. MySQL normally returns at least
        two result sets for a stored procedure call. The
        simple case is that the stored procedure contains a single
        SQL query, and it succeeds: you get two results, first the
        results of the embedded SQL query, and then the result
        of the call itself. If there are multiple SQL queries
        within the stored procedure, you get more than two result
        sets. Until you consume them all, you can’t start a
        new query on the connection. As above, you want to have
        a loop calling more_results()
        and store_next() to work your
        way through all of the result sets produced by the stored
        procedure call.
The following example demonstrates how to get information
    about the fields in a result set, such as the name of the field and
    the SQL type. This is
    examples/fieldinf.cpp:
#include "cmdline.h"
#include "printdata.h"
#include <iostream>
#include <iomanip>
using namespace std;
int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }
    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        // Get contents of main example table
        mysqlpp::Query query = con.query("select * from stock");
        mysqlpp::StoreQueryResult res = query.store();
        // Show info about each field in that table
        char widths[] = { 12, 22, 46 };
        cout.setf(ios::left);
        cout << setw(widths[0]) << "Field" <<
                setw(widths[1]) << "SQL Type" <<
                setw(widths[2]) << "Equivalent C++ Type" <<
                endl;
        for (size_t i = 0; i < sizeof(widths) / sizeof(widths[0]); ++i) {
            cout << string(widths[i] - 1, '=') << ' ';
        }
        cout << endl;
        
        for (size_t i = 0; i < res.field_names()->size(); i++) {
            // Suppress C++ type name outputs when run under dtest,
            // as they're system-specific.
            const char* cname = res.field_type(int(i)).name();
            mysqlpp::FieldTypes::value_type ft = res.field_type(int(i));
            ostringstream os;
            os << ft.sql_name() << " (" << ft.id() << ')';
            cout << setw(widths[0]) << res.field_name(int(i)).c_str() <<
                    setw(widths[1]) << os.str() <<
                    setw(widths[2]) << cname <<
                    endl;
        }
        cout << endl;
        // Simple type check
        if (res.field_type(0) == typeid(string)) {
            cout << "SQL type of 'item' field most closely resembles "
                    "the C++ string type." << endl;
        }
        // Tricky type check: the 'if' path shouldn't happen because the
        // description field has the NULL attribute.  We need to dig a
        // little deeper if we want to ignore this in our type checks.
        if (res.field_type(5) == typeid(string)) {
            cout << "Should not happen! Type check failure." << endl;
        }
        else if (res.field_type(5) == typeid(mysqlpp::sql_blob_null)) {
            cout << "SQL type of 'description' field resembles "
                    "a nullable variant of the C++ string type." << endl;
        }
        else {
            cout << "Weird: fifth field's type is now " <<
                    res.field_type(5).name() << endl;
            cout << "Did something recently change in resetdb?" << endl;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }
    return 0;
}
[2] SQLQueryParms is used as a
    stream only as an implementation detail within the library. End user
    code simply sees it as a std::vector
    derivative.
[3] By contrast, the
    Query methods that take an SSQLS do add quotes and
    escape strings implicitly. It can do this because SSQLS knows all
    the SQL code and data types, so it never has to guess whether
    quoting or escaping is appropriate.
[4] Unless you’re smarter than I am, you don’t immediately see why explicit manipulators are necessary. We can tell when quoting and escaping is not appropriate based on type, so doesn’t that mean we know when it is appropriate? Alas, no. For most data types, it is possible to know, or at least make an awfully good guess, but it’s a complete toss-up for C strings, const char*. A C string could be either a literal string of SQL code, or it can be a value used in a query. Since there’s no easy way to know and it would damage the library’s usability to mandate that C strings only be used for one purpose or the other, the library requires you to be explicit.
[5] One hopes the programmer knows.
[6] Yes, we’re aware that there is a feature in MySQL that lets you transfer row data in a binary form, but we don’t support this yet. We may, someday, probably as an extension to SSQLS. The only real reason to do so is to shave off some of the data translation overhead, which is typically neglibible in practice, swamped by the far greater disk and network I/O overheads inherent in use of a client-server database system like MySQL.
[7] SQL’s DECIMAL data type is a configurable-precision fixed-point number format. MySQL++ currently translates these to double, a floating-point data format, the closest thing available in the C++ type system. Since the main reason to use DECIMAL is to get away from the weird roundoff behavior of floating-point numbers, this could be viewed as a serious problem. The thing is, though, in all the years MySQL++ has been around, I don’t remember anyone actually complaining about it. Apparently there’s either no one using DECIMAL with MySQL++, or they’re ignoring any roundoff errors they get as a result. Until this wheel squeaks, it’s not likely to be greased. To fix this, we’ll have to create a new custom data type to hold such column values, which will be a lot of work for apparently little return.
[8] In version 2
      of MySQL++ and earlier, SQLTypeAdapter was
      called SQLString, but it was confusing
      because its name and the fact that it derived from
      std::string suggested that it was a
      general-purpose string type. MySQL++ even used it this way in a
      few places internally. In v3, we made it a simple base class and
      renamed it to reflect its proper limited
      function.
[9] SQLTypeAdapter
      doesn’t do quoting and escaping itself. That happens
      elsewhere, right at the point that the STA
      gets used to build a query.
[10] If you
      used MySQL++ before v3, String used to be
      called ColData. It was renamed because
      starting in v2.3, we began using it for holding more than just
      column data. I considered renaming it
      SQLString instead, but that would have
      confused old MySQL++ users to no end. Instead, I followed the
      example of Set, MySQL++’s specialized
      std::set variant.
[11] During the development of
      MySQL++ v3.0, I tried merging
      SQLTypeAdapter and
      String into a single class to take
      advantage of this. The resulting class gave the C++ compiler the
      freedom to tie itself up in knots, because it was then allowed to
      convert almost any data type to almost any other. You’d get
      a tangle of ambiguous data type conversion errors from the most
      innocent code.
[12] Don’t ping the server too often! It takes a tiny amount of processing capability to handle a ping, which can add up to a significant amount if done often enough by a client, or even just rarely by enough clients. Also, a lower ping frequency can let your program ride through some types of network faults — a switch reboot, for instance — without needing a reconnect. I like to ping the DB server no more often than half the connection timeout. With the default of 8 hours, then, I’d ping between every 4 and 7 hours.