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Difference between revisions of "Template:cpp/container/front"

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(added note about UB for empty containers)
 
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{{cpp/container/{{{1|}}}/title | front}}
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{{#vardefine:cont|{{{1|inplace_vector}}}}}<!--
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{{cpp/container/{{{1|}}}/navbar}}
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-->{{cpp/container/{{#var:cont}}/title|front}}
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{{ddcl list begin}}
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{{cpp/container/{{#var:cont}}/navbar}}
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{{ddcl list item | notes={{cpp/container/mark since c++11 | {{{1|}}} }} |
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{{dcl begin}}
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{{#switch:{{#var:cont}}
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|array=
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{{dcl|num=1|since=c++11|notes={{mark constexpr since c++17}}|
 
reference front();
 
reference front();
 
}}
 
}}
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{{ddcl list item | notes={{cpp/container/mark since c++11 | {{{1|}}} }} |
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{{dcl|num=2|since=c++11|notes={{mark constexpr since c++14}}|
 
const_reference front() const;
 
const_reference front() const;
 
}}
 
}}
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{{ddcl list end}}
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|vector=
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{{dcl|num=1|notes={{mark constexpr since c++20}}|
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reference front();
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}}
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{{dcl|num=2|notes={{mark constexpr since c++20}}|
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const_reference front() const;
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}}
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|inplace_vector=
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{{dcl|num=1|since=c++26|
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constexpr reference front();
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}}
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{{dcl|num=2|since=c++26|
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constexpr const_reference front() const;
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}}
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|
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{{dcl|num=1|since={{cpp/std|{{#var:cont}}}}|
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reference front();
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}}
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{{dcl|num=2|since={{cpp/std|{{#var:cont}}}}|
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const_reference front() const;
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}}
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}}
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{{dcl end}}
  
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Returns a reference to the first element in the container.  
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Returns a reference to the first element in the container.
  
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For a container {{tt|c}}, the expression {{c|c.front()}} is equivalent to {{c|*c.begin()}}.
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Calling {{tt|front}} on an empty container causes undefined behavior.
  
 
===Parameters===
 
===Parameters===
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===Return value===
 
===Return value===
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reference to the first element
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Reference to the first element.
  
 
===Complexity===
 
===Complexity===
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Constant
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Constant.
  
 
===Notes===
 
===Notes===
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Calling {{tt|front}} on an empty container is undefined.
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For a container {{tt|c}}, the expression {{c|c.front()}} is equivalent to {{c|*c.begin()}}.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
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| The following code uses {{tt|front}} to display the first element of a {{c|std::{{{1}}}<char>}}:
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|The following code uses {{tt|front}} to display the first element of a {{c|std::{{#var:cont}}<char{{#switch:{{#var:cont}}|inplace_vector|array=, 4}}>}}:
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| code=
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|code=
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#include <{{{1}}}>
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#include <cassert>
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#include <iostream>
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#include <{{#var:cont}}>
 
   
 
   
 
int main()
 
int main()
 
{
 
{
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     std::{{{1}}}<char> letters {'o', 'm', 'g', 'w', 't', 'f'};
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     std::{{#var:cont}}<char{{#switch:{{#var:cont}}|inplace_vector|array=, 4}}> letters{'a', 'b', 'c', 'd'};
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     assert(letters.front() == 'a');
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     if (!letters.empty()) {
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        std::cout << "The first character is: " << letters.front() << '\n';
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    } 
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}
 
}
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| output=
 
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The first character is o
 
 
}}
 
}}
  
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{{#ifeq: {{{1|}}} | forward_list ||
 
 
===See also===
 
===See also===
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{{dsc begin}}
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{{dcl list begin}}
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{{#switch:{{#var:cont}}
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{{dcl list template | cpp/container/dcl list back |{{{1|}}}}}
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|forward_list=
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{{dcl list end}}
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{{dsc inc|cpp/container/dsc before_begin|forward_list}}
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|
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{{dsc inc|cpp/container/dsc back|{{#var:cont}}}}
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{{dsc inc|cpp/container/dsc rend|{{#var:cont}}}}
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}}
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{{dsc inc|cpp/container/dsc begin|{{#var:cont}}}}
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{{#switch:{{#var:cont}}
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|array|vector|inplace_vector=
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{{dsc inc|cpp/container/dsc data|{{#var:cont}}}}
 
}}
 
}}
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{{dsc end}}

Latest revision as of 02:21, 16 August 2024

 
C++
 
Containers library
 
std::inplace_vector
Member types
Member functions
Non-member functions
 
constexpr reference front();
 (1) (since C++26)
constexpr const_reference front() const;
 (2) (since C++26)

Returns a reference to the first element in the container.

Calling front on an empty container causes undefined behavior.

Contents

[edit] Parameters

(none)

[edit] Return value

Reference to the first element.

[edit] Complexity

Constant.

[edit] Notes

For a container c, the expression c.front() is equivalent to *c.begin().

[edit] Example

The following code uses front to display the first element of a std::inplace_vector<char, 4>:

Run this code
#include <cassert>
#include <inplace_vector>
 
int main()
{
    std::inplace_vector<char, 4> letters{'a', 'b', 'c', 'd'};
    assert(letters.front() == 'a');
}

[edit] See also

 access the last element
(public member function of std::inplace_vector<T,N>) [edit]
returns a reverse iterator to the end
(public member function of std::inplace_vector<T,N>) [edit]
returns an iterator to the beginning
(public member function of std::inplace_vector<T,N>) [edit]
direct access to the underlying contiguous storage
(public member function of std::inplace_vector<T,N>) [edit]
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