NAME

std::list::emplace - std::list::emplace

Synopsis

template< class... Args > (since C++11)
iterator emplace( const_iterator pos, Args&&... args );

Inserts a new element into the container directly before pos.

The element is constructed through std::allocator_traits::construct, which uses
placement-new to construct the element in-place at a location provided by the
container.

The arguments args... are forwarded to the constructor as
std::forward<Args>(args).... args... may directly or indirectly refer to a value in
the container.

No iterators or references are invalidated.

Parameters

pos - iterator before which the new element will be constructed
args - arguments to forward to the constructor of the element

Type requirements

-
T (the container's element type) must meet the requirements of EmplaceConstructible.

Return value

Iterator pointing to the emplaced element.

Complexity

Constant.

Exceptions

If an exception is thrown (e.g. by the constructor), the container is left
unmodified, as if this function was never called (strong exception guarantee).

Example

// Run this code

#include <iostream>
#include <string>
#include <list>

struct A {
std::string s;
A(std::string str) : s(std::move(str)) { std::cout << " constructed\n"; }
A(const A& o) : s(o.s) { std::cout << " copy constructed\n"; }
A(A&& o) : s(std::move(o.s)) { std::cout << " move constructed\n"; }
A& operator=(const A& other) {
s = other.s;
std::cout << " copy assigned\n";
return *this;
}
A& operator=(A&& other) {
s = std::move(other.s);
std::cout << " move assigned\n";
return *this;
}
};

int main()
{
std::list<A> container;

std::cout << "construct 2 times A:\n";
A two { "two" };
A three { "three" };

std::cout << "emplace:\n";
container.emplace(container.end(), "one");

std::cout << "emplace with A&:\n";
container.emplace(container.end(), two);

std::cout << "emplace with A&&:\n";
container.emplace(container.end(), std::move(three));

std::cout << "content:\n";
for (const auto& obj : container)
std::cout << ' ' << obj.s;
std::cout << '\n';
}

Output:

construct 2 times A:
constructed
constructed
emplace:
constructed
emplace with A&:
copy constructed
emplace with A&&:
move constructed
content:
one two three

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2164 C++11 it was not clear whether the arguments can clarified
refer to the container