std::vector<T,Allocator>::emplace_back (3) - Linux Manuals

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

NAME

std::vector<T,Allocator>::emplace_back - std::vector<T,Allocator>::emplace_back

Synopsis

template< class... Args > (since C++11)
void emplace_back( Args&&... args ); (until C++17)
template< class... Args > (since C++17)
reference emplace_back( Args&&... args );

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.

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.

Return value

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

Complexity

Amortized constant.

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.

Notes

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.

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.
// Run this code

  #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(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::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 << 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.