std::deque emplace_back() method

since C++17

template< class... Args >
reference emplace_back( Args&&... args );

since C++11

// Non const version only
template< class... Args >
void emplace_back( Args&&... args );

Appends a new element to the end of the container.
The element is constructed through std::allocator_traits<Alloc>::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)....

Invalidation

All iterators, including the past-the-end iterator, are invalidated.
No references are invalidated.

Parameters

• 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

since C++17

A reference to the inserted element.

until C++17

(none)

Complexity

Constant - O(1).

Exceptions

If an exception is thrown, this function has no effect (strong exception guarantee).

Example

Main.cpp

#include <deque>
#include <string>
#include <cassert>
#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(other.name)), country(std::move(other.country)), year(other.year)
    {
        std::cout << "I am being moved.\n";
    }
    President& operator=(const President& other) = default;
};

int main()
{
    std::deque<President> elections;
    std::cout << "emplace_back:\n";
    auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    assert(ref.year == 1994 && "uses a reference to the created object (C++17)");

    std::deque<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 << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";
    }
    for (President const& president: reElections) {
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
    }
}

Output

emplace_back:
I am being constructed.

push_back:
I am being constructed.
I am being moved.

Contents:
Nelson Mandela was elected president of South Africa in 1994.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.