std::list<T,Allocator>:: emplace_back
|
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 ) ... .
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) |
(until C++17) |
|
A reference to the inserted element. |
(since C++17) |
Complexity
Constant.
Exceptions
If an exception is thrown for any reason, this function has no effect ( strong exception safety 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 <cassert> #include <iostream> #include <string> 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"; 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<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) |
|
|
(C++11)
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constructs element in-place
(public member function) |