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Difference between revisions of "cpp/thread/once flag"

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{{cpp/title|once_flag}}
 
{{cpp/title|once_flag}}
 
{{cpp/thread/navbar}}
 
{{cpp/thread/navbar}}
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{{ddcl | header=mutex | notes={{mark since c++11}} |
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{{ddcl | header=mutex | since=c++11 |
 
class once_flag;
 
class once_flag;
 
}}
 
}}
  
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The class {{tt|std::once_flag}} is a helper structure for {{c|std::call_once}}.
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The class {{tt|std::once_flag}} is a helper structure for {{lc|std::call_once}}.
  
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An object of type {{tt|std::once_flag}} that is passed to multiple calls to {{c|std::call_once}} allows those calls to coordinate with each other such that only one of the calls will actually run to completion.
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An object of type {{tt|std::once_flag}} that is passed to multiple calls to {{lc|std::call_once}} allows those calls to coordinate with each other such that only one of the calls will actually run to completion.
  
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{{tt|std::once_flag}} is noncopyable.
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{{tt|std::once_flag}} is neither copyable nor movable.
  
 
===Member functions===
 
===Member functions===
 
{{member | {{small|std::once_flag::}}once_flag |
 
{{member | {{small|std::once_flag::}}once_flag |
 
{{ddcl |
 
{{ddcl |
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once_flag();
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constexpr once_flag() noexcept;
 
}}
 
}}
  
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(none)
 
(none)
  
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===Exceptions===
 
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{{noexcept}}
 
 
}}
 
}}
  
 
===See also===
 
===See also===
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{{dcl list begin}}
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{{dsc begin}}
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{{dcl list template | cpp/thread/dcl list call_once}}
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{{dsc inc | cpp/thread/dsc call_once}}
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{{dcl list end}}
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{{dsc see c | c/thread/call_once | once_flag}}
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{{dsc end}}
  
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[[de:cpp/thread/once flag]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/thread/once flag]]
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[[fr:cpp/thread/once flag]]
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[[it:cpp/thread/once flag]]
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[[zh:cpp/thread/once flag]]
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Latest revision as of 06:55, 16 October 2021

 
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Defined in header <mutex>
class once_flag;
 (since C++11)

The class std::once_flag is a helper structure for std::call_once.

An object of type std::once_flag that is passed to multiple calls to std::call_once allows those calls to coordinate with each other such that only one of the calls will actually run to completion.

std::once_flag is neither copyable nor movable.

Contents

[edit] Member functions

std::once_flag::once_flag

constexpr once_flag() noexcept;

Constructs an once_flag object. The internal state is set to indicate that no function has been called yet.

Parameters

(none)


[edit] See also

(C++11)
 invokes a function only once even if called from multiple threads
(function template) [edit]
C documentation for once_flag
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