Difference between revisions of "Template:cpp/container/emplace back"
From cppreference.com
(LWG 2187) |
m (→Example: fmt) |
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===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | + | |The following code uses {{tt|emplace_back}} to append an object of type {{tt|President}} to a {{c|std::{{{1}}}}}. It demonstrates how {{tt|emplace_back}} forwards parameters to the {{tt|President}} constructor and shows how using {{tt|emplace_back}} avoids the extra copy or move operation required when using {{tt|push_back}}. | |
| − | + | |code = | |
#include <{{{1}}}> | #include <{{{1}}}> | ||
| − | |||
#include <cassert> | #include <cassert> | ||
#include <iostream> | #include <iostream> | ||
| + | #include <string> | ||
struct President | struct President | ||
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std::cout << "I am being constructed.\n"; | std::cout << "I am being constructed.\n"; | ||
} | } | ||
| + | |||
President(President&& other) | President(President&& other) | ||
: name(std::move(other.name)), country(std::move(other.country)), year(other.year) | : name(std::move(other.name)), country(std::move(other.country)), year(other.year) | ||
| Line 75: | Line 76: | ||
std::cout << "I am being moved.\n"; | std::cout << "I am being moved.\n"; | ||
} | } | ||
| + | |||
President& operator=(const President& other) = default; | President& operator=(const President& other) = default; | ||
}; | }; | ||
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std::cout << "\nContents:\n"; | std::cout << "\nContents:\n"; | ||
| − | for (President const& president: elections) | + | for (President const& president: elections) |
std::cout << president.name << " was elected president of " | std::cout << president.name << " was elected president of " | ||
<< president.country << " in " << president.year << ".\n"; | << president.country << " in " << president.year << ".\n"; | ||
| − | + | ||
| − | for (President const& president: reElections) | + | for (President const& president: reElections) |
std::cout << president.name << " was re-elected president of " | std::cout << president.name << " was re-elected president of " | ||
<< president.country << " in " << president.year << ".\n"; | << president.country << " in " << president.year << ".\n"; | ||
| − | |||
} | } | ||
| − | + | |output= | |
emplace_back: | emplace_back: | ||
I am being constructed. | I am being constructed. | ||
Revision as of 12:35, 30 May 2023
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)....
| Information on iterator invalidation is copied from here |
Contents |
Parameters
| args | - | arguments to forward to the constructor of the element |
| Type requirements
| ||
Return value
|
(none) |
(until C++17) |
|
A reference to the inserted element. |
(since C++17) |
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::{{{1}}}. 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 <{{{1}}}> #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::{{{1}}}<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::{{{1}}}<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 of std::{{{1}}})
| |
| constructs element in-place (public member function of std::{{{1}}})
|
