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

Revision as of 14:32, 2 July 2012

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

For example, std::vector::resize occasionally allocates new storage and then moves or copies 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.

Parameters

Return value

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

Exceptions

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);
}

Non-throwing move constructor called
Throwing copy constructor called

Complexity

Constant

See also