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    std::atomic<T>::atomic

    From cppreference.com
     
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    std::atomic
    Member functions
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    Specialized for integral,
    floating-point     (C++20)
    and pointer types
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    pointer types only
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    Specialized for integral types only
     
    constexpr atomic() noexcept(std::is_nothrow_default_constructible_v<T>);
    		(1) (since C++11) 
    constexpr atomic( T desired ) noexcept;
    		(2) (since C++11) 
    atomic( const atomic& ) = delete;
    		(3) (since C++11)

    Constructs a new atomic variable.

    1) Value-initializes the underlying object (i.e. with T()). The initialization is not atomic.
    This overload participates in overload resolution only if std::is_default_constructible_v<T> is true.
    2) Initializes the underlying object with desired. The initialization is not atomic.
    3) Atomic variables are not CopyConstructible.

    Parameters

    desired - value to initialize with

    Note

    In libstdc++, libc++, LWG issue 2334 was only fixed in C++20 and later. Prior to the fix, the above implementations all initialize the underlying object by default-initialization.

    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 4169 C++11 std::is_default_constructible_v could report false positive the default constructor is constrained
    LWG 2334 C++11 The default constructor does not create a T object Corrected

    See also

    (C++11)(deprecated in C++20)
    		 non-atomic initialization of a default-constructed atomic object
    (function template) [edit]