NAME

std::move_if_noexcept - std::move_if_noexcept

Synopsis

Defined in header <utility>
template< class T >

typename std::conditional<
!std::is_nothrow_move_constructible<T>::value && (since C++11)
std::is_copy_constructible<T>::value, (until C++14)
const T&,
T&&

>::type move_if_noexcept(T& x) noexcept;
template< class T >

constexpr typename std::conditional<
!std::is_nothrow_move_constructible<T>::value &&
std::is_copy_constructible<T>::value, (since C++14)
const T&,
T&&

>::type move_if_noexcept(T& x) noexcept;

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.

Parameters

x - the object to be moved or copied

Return value

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

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)

Example

// Run this code

#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;
Good g2 = std::move_if_noexcept(g);
Bad b2 = std::move_if_noexcept(b);
}

Output:

Non-throwing move constructor called
Throwing copy constructor called

Complexity

Constant