std::move_if_noexcept

From Cppreference

Obtains an rvalue reference to its argument if its move constructor does not throw exceptions, otherwise obtains an lvalue reference to its argument. Typically used to combine move semantics with strong exception guarantee.

For example, std::vector::resize() allocates new storage, and then moves or copies elements from old storage to new storage. If an exception occurs during this, 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.

[edit] Parameters

[edit] Return value

std::move(x)

[edit] Exceptions

[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&) {};
};
 
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

[edit] Complexity

Constant

[edit] See also