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 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.
Parameters
args | - | arguments to forward to the constructor of the element |
Type requirements | ||
- T (the container's element type) must meet the requirements of EmplaceConstructible . |
Return value
(none).
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::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 <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 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) |
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