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Difference between revisions of "cpp/atomic/ATOMIC FLAG INIT"

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{{title|ATOMIC_FLAG_INIT}}
 
{{title|ATOMIC_FLAG_INIT}}
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{{cpp/atomic/navbar}}
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{{ddcl list begin}}
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{{ddcl|header=atomic|since=c++11|
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{{ddcl list header | atomic}}
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{{ddcl list item |
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#define ATOMIC_FLAG_INIT /* implementation-defined */
 
#define ATOMIC_FLAG_INIT /* implementation-defined */
 
}}
 
}}
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{{ddcl list end}}
 
  
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Defines the expression which can be used to initialize {{c|std::atomic_flag}} to clear state. If the flag has static storage duration, this initialization is static.
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Defines the initializer which can be used to initialize {{lc|std::atomic_flag}} to clear (false) state in the form {{c|1=std::atomic_flag v = ATOMIC_FLAG_INIT;}}. It is unspecified if it can be used with other initialization contexts.
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If the flag has is a [[cpp/language/object#Subobjects|complete object]] with {{lsd|cpp/language/storage_duration#Static storage duration}}, this [[cpp/language/initialization#Static initialization|initialization is static]].
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{{rev begin}}
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{{rev|until=c++20|
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This is the only way to initialize {{lc|std::atomic_flag}} to a definite value: the value held after any other initialization is unspecified.
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}}
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{{rev|since=c++20|
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This macro is no longer needed since default constructor of {{lc|std::atomic_flag}} initializes it to clear state. It is kept for the compatibility with C.
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}}
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{{rev end}}
  
 
===Example===
 
===Example===
 
{{example
 
{{example
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|
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| code=
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|code=
 
#include <atomic>
 
#include <atomic>
  
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std::atomic_flag static_flag = ATOMIC_FLAG_INIT;
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std::atomic_flag static_flag = ATOMIC_FLAG_INIT; // static initialization,
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// guaranteed to be available during dynamic initialization of static objects.
  
 
int main()
 
int main()
 
{
 
{
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     std::atomic_flag automatic_flag = ATOMIC_FLAG_INIT;
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     std::atomic_flag automatic_flag = ATOMIC_FLAG_INIT; // guaranteed to work
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    std::atomic_flag another_flag(ATOMIC_FLAG_INIT);
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//    std::atomic_flag another_flag(ATOMIC_FLAG_INIT); // unspecified
 
}
 
}
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| output=
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|output=
 
}}
 
}}
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===Defect reports===
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{{dr list begin}}
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{{dr list item|wg=lwg|dr=2159|std=C++11|before=it was unclear whether {{tt|ATOMIC_FLAG_INIT}}<br>can be used with other initialization contexts|after=other usages are<br>not guaranteed}}
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{{dr list item|wg=lwg|dr=3659|std=C++20|before={{tt|ATOMIC_FLAG_INIT}} was deprecated, but needed in C on some platforms|after=it is undeprecated}}
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{{dr list end}}
  
 
===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/atomic/dcl list atomic_flag}}
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{{dsc inc|cpp/atomic/dsc atomic_flag}}
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{{dcl list end}}
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{{dsc see c|c/atomic/ATOMIC_FLAG_INIT}}
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{{dsc end}}
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 18:13, 17 October 2024

 
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Defined in header <atomic>
#define ATOMIC_FLAG_INIT /* implementation-defined */
 (since C++11)

Defines the initializer which can be used to initialize std::atomic_flag to clear (false) state in the form std::atomic_flag v = ATOMIC_FLAG_INIT;. It is unspecified if it can be used with other initialization contexts.

If the flag has is a complete object with static storage duration, this initialization is static.

This is the only way to initialize std::atomic_flag to a definite value: the value held after any other initialization is unspecified.

(until C++20)

This macro is no longer needed since default constructor of std::atomic_flag initializes it to clear state. It is kept for the compatibility with C.

(since C++20)

[edit] Example

Run this code
#include <atomic>
 
std::atomic_flag static_flag = ATOMIC_FLAG_INIT; // static initialization,
// guaranteed to be available during dynamic initialization of static objects.
 
int main()
{
    std::atomic_flag automatic_flag = ATOMIC_FLAG_INIT; // guaranteed to work
//    std::atomic_flag another_flag(ATOMIC_FLAG_INIT); // unspecified
}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2159 C++11 it was unclear whether ATOMIC_FLAG_INIT
can be used with other initialization contexts 		other usages are
not guaranteed
LWG 3659 C++20 ATOMIC_FLAG_INIT was deprecated, but needed in C on some platforms it is undeprecated

[edit] See also

(C++11)
 the lock-free boolean atomic type
(class) [edit]
C documentation for ATOMIC_FLAG_INIT
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