Difference between revisions of "Template:cpp/container/emplace back"
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| The following code uses {{tt|emplace_back}} to add an element to a {{c|std::{{{1}}}}} of an arbitrary class ({{tt|President}}). It uses the parameters of {{tt|President}} constructor. It also shows how using {{tt|emplace_back}} saves the extra copy or move operation required when using {{tt|push_back}}. | | The following code uses {{tt|emplace_back}} to add an element to a {{c|std::{{{1}}}}} of an arbitrary class ({{tt|President}}). It uses the parameters of {{tt|President}} constructor. It also shows how using {{tt|emplace_back}} saves the extra copy or move operation required when using {{tt|push_back}}. | ||
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Revision as of 18:20, 19 April 2012
Template:cpp/container//sidebar
| 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.
| Information on iterator invalidation is copied from here |
Contents |
Parameters
| args | - | arguments to forward to the constructor of the element |
Return value
(none)
Complexity
Constant.
Example
The following code uses emplace_back to add an element to a std::{{{1}}} 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.
Run this code
#include <{{{1}}}> #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::{{{1}}}<President> elections; std::cout << "emplace_back:\n"; elections.emplace_back("Nelson Mandela", "South Africa", 1994); std::{{{1}}}<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.
