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ATOMIC_VAR_INIT

From cppreference.com
< cpp‎ | atomic
Defined in header <atomic>
#define ATOMIC_VAR_INIT(value) /* implementation-defined */

Expands to an expression which can be used to initialize an std::atomic object that can be initialized from value. If the atomic object has static storage duration, this initialization is constant initialization.

[edit] Notes

Accessing the variable during initialization from another thread, even through an atomic operation, is a data race (it may happen if the address is immediately passed to another thread with a std::memory_order_relaxed operation)

This macro is primarily provided for compatibility with C; it behaves the same as the constructor of std::atomic.

[edit] Example

#include <atomic>
#include <iostream>
 
int main()
{
    std::atomic<int> a = ATOMIC_VAR_INIT(1);
    // std::atomic<int> a(1);   // C++-only alternative
    std::cout << "Initialized std::atomic<int> as: " << a << '\n';
}

Output:

Initialized std::atomic<int> as: 1

[edit] See also

non-atomic initialization of a default-constructed atomic object
(function template) [edit]
constructs an atomic object
(public member function of std::atomic) [edit]