Difference between revisions of "cpp/utility/move if noexcept"

Latest revision as of 13:10, 13 July 2024

std::move_if_noexcept obtains an rvalue reference to its argument if its move constructor does not throw exceptions or if there is no copy constructor (move-only type), otherwise obtains an lvalue reference to its argument. It is typically used to combine move semantics with strong exception guarantee.

The return type of std::move_if_noexcept is:

• T&& if std::is_nothrow_move_constructible<T>::value || !std::is_copy_constructible<T>::value is true.
• Otherwise, const T&.

[edit] Parameters

[edit] Return value

std::move(x) or x, depending on exception guarantees.

[edit] Complexity

Constant.

[edit] Notes

This is used, for example, by std::vector::resize, which may have to allocate new storage and then move or copy elements from old storage to new storage. If an exception occurs during this operation, std::vector::resize undoes everything it did to this point, which is only possible if std::move_if_noexcept was used to decide whether to use move construction or copy construction (unless copy constructor is not available, in which case move constructor is used either way and the strong exception guarantee may be waived).

[edit] Example

#include <iostream>
#include <utility>
 
struct Bad
{
    Bad() {}
    Bad(Bad&&) // may throw
    {
        std::cout << "Throwing move constructor called\n";
    }
    Bad(const Bad&) // may throw as well
    {
        std::cout << "Throwing copy constructor called\n";
    }
};
 
struct Good
{
    Good() {}
    Good(Good&&) noexcept // will NOT throw
    {
        std::cout << "Non-throwing move constructor called\n";
    }
    Good(const Good&) noexcept // will NOT throw
    {
        std::cout << "Non-throwing copy constructor called\n";
    }
};
 
int main()
{
    Good g;
    Bad b;
    [[maybe_unused]] Good g2 = std::move_if_noexcept(g);
    [[maybe_unused]] Bad b2 = std::move_if_noexcept(b);
}

Non-throwing move constructor called
Throwing copy constructor called

[edit] See also