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std::list<T,Allocator>::emplace

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

(since C++11)

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

#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 refer to the container	clarified

insert	inserts elements (public member function)
emplace_back (C++11)	constructs an element in-place at the end (public member function)