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

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    std::atomic
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    member only of atomic<Integral > specializations
    and atomic<Floating > specializations(since C++20)
    		 
    T fetch_sub( T arg, std::memory_order order =
                        std::memory_order_seq_cst ) noexcept;
    		(1) (since C++11) 
    T fetch_sub( T arg, std::memory_order order =
                        std::memory_order_seq_cst ) volatile noexcept;
    		(2) (since C++11)
    member only of atomic<T*> partial specialization
    		 
    T* fetch_sub( std::ptrdiff_t arg,
              std::memory_order order =
                  std::memory_order_seq_cst ) noexcept;
    		(3) (since C++11) 
    T* fetch_sub( std::ptrdiff_t arg,
              std::memory_order order =
                  std::memory_order_seq_cst ) volatile noexcept;
    		(4) (since C++11)

    Atomically replaces the current value with the result of arithmetic subtraction of the value and arg. That is, it performs atomic post-decrement. The operation is read-modify-write operation. Memory is affected according to the value of order.

    1,2) For signed integral types, arithmetic is defined to use two’s complement representation. There are no undefined results.

    For floating-point types, the floating-point environment in effect may be different from the calling thread's floating-point environment. The operation need not be conform to the corresponding std::numeric_limits traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the operation otherwise has no undefined behavior.

    		(since C++20)
    3,4) The result may be an undefined address, but the operation otherwise has no undefined behavior.
    If T is not a complete object type, the program is ill-formed.


    It is deprecated if std::atomic<T>::is_always_lock_free is false and overload (2) or (4) participates in overload resolution.

    		(since C++20)

    Parameters

    arg - the other argument of arithmetic subtraction
    order - memory order constraints to enforce

    Return value

    The value immediately preceding the effects of this function in the modification order of *this.

    Defect reports

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

    DR Applied to Behavior as published Correct behavior
    P0558R1 C++11 arithmetic permitted on pointers to (possibly cv-qualified) void or function made ill-formed

    See also