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std::atomic<T>::atomic - cppreference.com

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(1)

atomic() noexcept = default;

 (since C++11)
(until C++20)

constexpr atomic() noexcept(std::is_nothrow_default_constructible_v<T>);

 (since C++20)

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.

Parameters

desired - value to initialize with

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-initialized does not contain T object Corrected