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Difference between revisions of "Template:cpp/container/emplace back"

From cppreference.com
m (Text replace - "par req concept" to "par req named")
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===Notes===
 
===Notes===
 +
Since reallocation may take place, {{tt|std::vector<T>::emplace_back}} requires the element to be move constructible.
 +
 
The specialization [[cpp/container/vector_bool|std::vector<bool>]] did not have {{tt|emplace_back()}} member until C++14.
 
The specialization [[cpp/container/vector_bool|std::vector<bool>]] did not have {{tt|emplace_back()}} member until C++14.
  

Revision as of 19:22, 29 October 2018

 
C++
 
Containers library
 
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)

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)....

Information on iterator invalidation is copied from here

Contents

Parameters

args - arguments to forward to the constructor of the element
Type requirements


Return value

(none) (until C++17)
A reference to the inserted element. (since C++17)

Complexity

Constant.

Exceptions

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


Example

The following code uses emplace_back to append an object of type President to a std::{{{1}}}. 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 <{{{1}}}>
#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(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::{{{1}}}<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
    std::{{{1}}}<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.

See also

 adds an element to the end
(public member function of std::{{{1}}}) [edit]
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