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std::list::emplace_back

From cppreference.com
template< class... Args >
void emplace_back( Args&&... args );
(since C++11)

Appends a new element to the end of the container. The element is constructed in-place, i.e. no copy or move operations are performed. The constructor of the element is called with exactly the same arguments, as supplied to the function.

No iterators or references are invalidated.

Contents

[edit] Parameters

args - arguments to forward to the constructor of the element

[edit] Return value

(none)

[edit] Complexity

Constant.

[edit] Example

The following code uses emplace_back to add an element to a std::list of an arbitrary class (President). It uses the parameters of President constructor. It also shows how using emplace_back saves the extra copy or move operation required when using push_back.

#include <list>
#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::list<President> elections;
  std::cout << "emplace_back:\n";
  elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
  std::list<President> reElections;
  std::cout << "\npush_back:\n";
  reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
 
  std::cout << "\nContents:\n";
  for (President & president: elections)
  {
    std::cout << president.name << " was elected president of "
      << president.country << " in " << president.year << ".\n";
  }
  for (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.

[edit] See also

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