clang-tidy - bugprone-move-forwarding-reference — Extra Clang Tools 7 documentation

# Extra Clang Tools 7 documentation

## clang-tidy - bugprone-move-forwarding-reference

# bugprone-move-forwarding-reference

Warns if `std::move` is called on a forwarding reference, for example:

>     template <typename T>
>     void foo(T&& t) {
>       bar(std::move(t));
>     }

Forwarding references should typically be passed to `std::forward` instead of `std::move`, and this is the fix that will be suggested.

(A forwarding reference is an rvalue reference of a type that is a deduced function template argument.)

In this example, the suggested fix would be

>     bar(std::forward<T>(t));

## Background

Code like the example above is sometimes written with the expectation that `T&&` will always end up being an rvalue reference, no matter what type is deduced for `T`, and that it is therefore not possible to pass an lvalue to `foo()`. However, this is not true. Consider this example:

>     std::string s = "Hello, world";
>     foo(s);

This code compiles and, after the call to `foo()`, `s` is left in an indeterminate state because it has been moved from. This may be surprising to the caller of `foo()` because no `std::move` was used when calling `foo()`.

The reason for this behavior lies in the special rule for template argument deduction on function templates like `foo()` – i.e. on function templates that take an rvalue reference argument of a type that is a deduced function template argument. (See section [temp.deduct.call]/3 in the C++11 standard.)

If `foo()` is called on an lvalue (as in the example above), then `T` is deduced to be an lvalue reference. In the example, `T` is deduced to be `std::string &`. The type of the argument `t` therefore becomes `std::string& &&`; by the reference collapsing rules, this collapses to `std::string&`.

This means that the `foo(s)` call passes `s` as an lvalue reference, and `foo()` ends up moving `s` and thereby placing it into an indeterminate state.