W3cubDocs

/C++

std::atomic<T>::fetch_sub

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

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.

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

For T* types, the result may be an undefined address, but the operation otherwise has no undefined behavior. The program is ill-formed if T is not an object type.

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.

The volatile-qualified versions are deprecated if std::atomic<T>::is_always_lock_free is false.

 (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 cv void or function made ill-formed

See also

(C++11)(C++11)
 subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic
(function template)
increments or decrements the atomic value by one
(public member function)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/atomic/atomic/fetch_sub