std::atomic_ref<T>::operator+=,-=

Provided only when `T` is an arithmetic type other than cv bool or a pointer-to-object type
value_type operator+=( difference_type arg ) const noexcept;	(1)	(constexpr since C++26)
value_type operator-=( difference_type arg ) const noexcept;	(2)	(constexpr since C++26)

Atomically replaces the current value referenced by *ptr with the result of computation involving the previous value and arg. These operations are read-modify-write operations.

1) operator+= performs atomic addition. Equivalent to return fetch_add(arg) + arg;.

2) operator-= performs atomic subtraction. Equivalent to return fetch_sub(arg) - arg;.

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.
For pointer types, the result may be an undefined address, but the operation otherwise has no undefined behavior.
- If std::remove_pointer_t<T> is not a complete object type, the program is ill-formed.

These overloads participate in overload resolution only if std::is_const_v<T> is false.

Parameters

arg

the argument for the arithmetic operation

Return value

The resulting value (that is, the result of applying the corresponding binary operator to the value referenced by *ptr immediately preceding the effects of the corresponding member function).

Notes

Unlike most compound assignment operators, the compound assignment operators for atomic_ref return a copy of the stored value instead of a reference to arg.

Example

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 3508 (P3323R1)	C++20	compound assignment operators were meaningless for constT	constrained to accept only non-const `T`

fetch_add	atomically adds the argument to the value stored in the referenced object and obtains the value held previously (public member function)
fetch_sub	atomically subtracts the argument from the value stored in the referenced object and obtains the value held previously (public member function)
operator++operator++(int)operator--operator--(int)	atomically increments or decrements the referenced object by one (public member function)
operator&=operator\|=operator^=	atomically performs bitwise AND, OR, XOR with the referenced value (public member function)