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

From cppreference.com
< cpp‎ | container‎ | vector
 
 
 
 
template< class... Args >
iterator emplace( const_iterator pos, Args&&... args );
(since C++11)
(until C++20)
template< class... Args >
constexpr iterator emplace( const_iterator pos, Args&&... args );
(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 the new size() is greater than capacity(), all iterators and references are invalidated. Otherwise, only the iterators and references before the insertion point remain valid. The past-the-end iterator is also invalidated.

Contents

[edit] 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.

[edit] Return value

Iterator pointing to the emplaced element.

[edit] Complexity

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

[edit] 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

#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

[edit] 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

[edit] See also

inserts elements
(public member function) [edit]
constructs an element in-place at the end
(public member function) [edit]