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Difference between revisions of "Template:cpp/container/emplace back"

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{{cpp/container/{{{1|}}}/title|emplace_back}}
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{{#vardefine:cont|{{{1|vector}}}}}<!--
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{{cpp/container/{{{1|}}}/navbar}}
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-->{{cpp/container/{{#var:cont}}/title|emplace_back}}
 +
{{cpp/container/{{#var:cont}}/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
 
{{dcl rev multi
 
{{dcl rev multi
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template< class... Args >
 
template< class... Args >
 
void emplace_back( Args&&... args );
 
void emplace_back( Args&&... args );
−
|since2=c++17|dcl2=
+
|since2=c++17|notes2={{#ifeq:{{#var:cont}}|vector|{{mark constexpr since c++20}}}}|dcl2=
 
template< class... Args >
 
template< class... Args >
 
reference emplace_back( Args&&... args );
 
reference emplace_back( Args&&... args );
−
|since3={{#ifeq:{{{1}}}|vector|c++20}}|dcl3={{#ifeq:{{{1}}}|vector|
 
−
template< class... Args >
 
−
constexpr reference emplace_back( Args&&... args );
 
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}}
 
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
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Appends a new element to the end of the container. The element is constructed through {{lc|std::allocator_traits::construct}}, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments {{c|args...}} are forwarded to the constructor as {{c|std::forward<Args>(args)...}}.
 
Appends a new element to the end of the container. The element is constructed through {{lc|std::allocator_traits::construct}}, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments {{c|args...}} are forwarded to the constructor as {{c|std::forward<Args>(args)...}}.
  
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{{cpp/container/note_iterator_invalidation|{{{1|}}}|emplace_back}}
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{{cpp/container/note_iterator_invalidation|{{#var:cont}}|emplace_back}}
  
 
===Parameters===
 
===Parameters===
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{{par|args|arguments to forward to the constructor of the element}}
 
{{par|args|arguments to forward to the constructor of the element}}
 
{{par hreq}}
 
{{par hreq}}
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{{#switch:{{{1|}}}|deque|list=
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{{#switch:{{#var:cont}}|deque|list=
 
{{par req named|T (the container's element type)|EmplaceConstructible}}
 
{{par req named|T (the container's element type)|EmplaceConstructible}}
 
|vector=
 
|vector=
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===Complexity===
 
===Complexity===
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{{#ifeq:{{{1|}}}|vector|Amortized constant.|Constant.}}
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{{#ifeq:{{#var:cont}}|vector|Amortized constant.|Constant.}}
  
 
===Exceptions===
 
===Exceptions===
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If an exception is thrown, this function has no effect (strong exception guarantee).
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{{cpp/strong exception safety guarantee|plural=no}}
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{{#ifeq: {{{1|}}}|vector|
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{{#ifeq: {{#var:cont}}|vector|
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If {{tt|T}}'s move constructor is not {{c|noexcept}} and is not {{named req|CopyInsertable}} into {{tt|*this}}, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.
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If the move constructor of {{tt|T}} is not {{c/core|noexcept}} and is not {{named req|CopyInsertable}} into {{c|*this}}, {{tt|vector}} will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.
 
}}
 
}}
−
{{#ifeq:{{{1|}}}|vector|
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{{#ifeq:{{#var:cont}}|vector|
  
 
===Notes===
 
===Notes===
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===Example===
 
===Example===
 
{{example
 
{{example
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|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}}.
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|The following code uses {{tt|emplace_back}} to append an object of type {{tt|President}} to a {{lc|std::{{#var:cont}}}}. 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 {{lc|push_back}}.
 
|code=
 
|code=
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#include <{{{1}}}>
+
#include <{{#var:cont}}>
 
#include <cassert>
 
#include <cassert>
 
#include <iostream>
 
#include <iostream>
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int main()
 
int main()
 
{
 
{
−
     std::{{{1}}}<President> elections;
+
     std::{{#var:cont}}<President> elections;
 
     std::cout << "emplace_back:\n";
 
     std::cout << "emplace_back:\n";
 
     auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
     auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
     assert(ref.year == 1994 && "uses a reference to the created object (C++17)");
 
     assert(ref.year == 1994 && "uses a reference to the created object (C++17)");
  
−
     std::{{{1}}}<President> reElections;
+
     std::{{#var:cont}}<President> reElections;
 
     std::cout << "\npush_back:\n";
 
     std::cout << "\npush_back:\n";
 
     reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
 
     reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
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===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc|cpp/container/dsc push_back|{{{1|}}}}}
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{{dsc inc|cpp/container/dsc push_back|{{#var:cont}}}}
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{{dsc inc|cpp/container/dsc emplace|{{{1|}}}}}
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{{dsc inc|cpp/container/dsc emplace|{{#var:cont}}}}
 
{{dsc end}}
 
{{dsc end}}

Latest revision as of 17:49, 29 August 2024

 
C++
 
Containers library
 
std::vector
Member types
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)
 
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)
(constexpr since C++20)

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)....

If after the operation the new size() is greater than old capacity() a reallocation takes place, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise only the end() iterator is invalidated.

Contents

[edit] Parameters

args - arguments to forward to the constructor of the element
Type requirements
-
T (the container's element type) must meet the requirements of MoveInsertable and EmplaceConstructible.

[edit] Return value

(none)

(until C++17)

A reference to the inserted element.

(since C++17)

[edit] Complexity

Amortized constant.

[edit] Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee). If the move constructor of T is not noexcept and is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

Notes

Since reallocation may take place, emplace_back requires the element type to be MoveInsertable for vectors.

[edit] Example

The following code uses emplace_back to append an object of type President to a std::vector. 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 <vector>
#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::vector<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::vector<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.

[edit] See also

 adds an element to the end
(public member function of std::vector<T,Allocator>) [edit]
(C++11)
 constructs element in-place
(public member function of std::vector<T,Allocator>) [edit]
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