std::list<T,Allocator>::emplace_back - cppreference.com (original) (raw)

| 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.

[edit] Parameters

[edit] Return value

[edit] Complexity

Constant.

[edit] Exceptions

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

[edit] 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.

#include #include #include #include   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 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 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.

[edit] See also

| | adds an element to the end (public member function) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | constructs element in-place (public member function) [edit] |