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

From cppreference.net

C++
Containers library
std::list
Member functions
Non-member functions
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20)
Deduction guides (C++17)
template < class ... Args >
void emplace_back ( Args && ... args ) ;
(until C++17)
template < class ... Args >
reference emplace_back ( Args && ... args ) ;
(since C++17)
(constexpr since C++26)

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 ) ... .

No iterators or references are invalidated.

Contents

Parameters

args - arguments to forward to the constructor of the element
Type requirements
-
If T is not EmplaceConstructible into list from args... , the behavior is undefined.

Return value

(none)

(until C++17)

A reference to the inserted element.

(since C++17)

Complexity

Constant.

Exceptions

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

Example

The following code uses emplace_back to append an object of type President to a std::list . 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 .

Run this code 
#include <list>
#include <cassert>
#include <iostream>
#include <string>
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::list<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::list<President> reElections;
    std::cout << "\npush_back:\n";
    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
    std::cout << "\nContents:\n";
    for (const President& president: elections)
        std::cout << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";
    for (const President& 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.

See also

adds an element to the end
(public member function)
(C++11)
constructs element in-place
(public member function)