< cpp‎ | container‎ | vector
template< class... Args >
void emplace_back( Args&&... args );
(since C++11)
(until C++17)
template< class... Args >
reference emplace_back( Args&&... args );
(since C++17)
(until C++20)
template< class... Args >
constexpr reference emplace_back( Args&&... args );
(since C++20)

Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments args... are forwarded to the constructor as std::forward<Args>(args)....

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.


[edit] Parameters

args - arguments to forward to the constructor of the element
Type requirements
T (the container's element type) must meet the requirements of MoveInsertable and EmplaceConstructible.

[edit] Return value


(until C++17)

A reference to the inserted element.

(since C++17)

[edit] Complexity

Amortized constant.

[edit] Exceptions

If an exception is thrown, this function has no effect (strong exception guarantee). If T's move constructor is not noexcept and is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.


Since reallocation may take place, emplace_back requires the element type to be MoveInsertable for vectors.

The specialization std::vector<bool> did not have emplace_back() member until C++14.

[edit] Example

The following code uses emplace_back to append an object of type President to a std::vector. It demonstrates how emplace_back forwards parameters to the President constructor and shows how using emplace_back avoids the extra copy or move operation required when using push_back.

#include <vector>
#include <string>
#include <iostream>
struct President
    std::string name;
    std::string country;
    int year;
    President(std::string p_name, std::string p_country, int p_year)
        : name(std::move(p_name)), country(std::move(p_country)), year(p_year)
        std::cout << "I am being constructed.\n";
    President(President&& other)
        : name(std::move(, country(std::move(, year(other.year)
        std::cout << "I am being moved.\n";
    President& operator=(const President& other) = default;
int main()
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    std::vector<President> reElections;
    std::cout << "\npush_back:\n";
    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
    std::cout << "\nContents:\n";
    for (President const& president: elections) {
        std::cout << << " was elected president of "
                  << << " in " << president.year << ".\n";
    for (President const& president: reElections) {
        std::cout << << " was re-elected president of "
                  << << " in " << president.year << ".\n";


I am being constructed.
I am being constructed.
I am being moved.
Nelson Mandela was elected president of South Africa in 1994.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

[edit] See also

adds an element to the end
(public member function) [edit]