NAME¶

std::vector::emplace - std::vector::emplace

Synopsis¶

template< class... Args > (since C++11)
iterator emplace( const_iterator pos, Args&&... args ); (constexpr since C++20)

Inserts a new element into the container directly before pos.

The element is constructed through std::allocator_traits::construct, which typically
uses placement-new to construct the element in-place at a location provided by the
container. However, if the required location has been occupied by an existing
element, the inserted element is constructed at another location at first, and then
move assigned into the required location.

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.

If after the operation the new size() is greater than old capacity() a reallocation
takes place, in which case all iterators (including the end() iterator) and all
references to the elements are invalidated. Otherwise, only the iterators and
references before the insertion point remain valid.

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 MoveAssignable,
MoveInsertable and EmplaceConstructible.

Return value¶

Iterator pointing to the emplaced element.

Complexity¶

Linear in the distance between pos and end of the container.

Exceptions¶

If an exception is thrown other than by the copy constructor, move constructor,
assignment operator, or move assignment operator of the value type, or if an
exception is thrown while emplace is used to insert a single element at the end and
the value type is either CopyInsertable or nothrow move constructible, there are no
effects (strong exception guarantee).

Otherwise, the effects are unspecified.

Example¶

// Run this code

#include <iostream>
#include <string>
#include <vector>

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::vector<A> container;
// reserve enough place so vector does not have to resize
container.reserve(10);
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

See also¶

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