# std::atomic<T>::operator+=,-=

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< cpp‎ | atomic‎ | atomic

member only of `atomic<Integral`﻿`>` specializations and `atomic<Floating`﻿`>` specializations(since C++20) |  |
T operator+=( T arg ) noexcept; | (1) | (since C++11)
T operator+=( T arg ) volatile noexcept; | (2) | (since C++11)
T operator-=( T arg ) noexcept; | (3) | (since C++11)
T operator-=( T arg ) volatile noexcept; | (4) | (since C++11)
member only of `atomic<T*>` partial specialization |  |
T* operator+=( std::ptrdiff_t arg ) noexcept; | (5) | (since C++11)
T* operator+=( std::ptrdiff_t arg ) volatile noexcept; | (6) | (since C++11)
T* operator-=( std::ptrdiff_t arg ) noexcept; | (7) | (since C++11)
T* operator-=( std::ptrdiff_t arg ) volatile noexcept; | (8) | (since C++11)

Atomically replaces the current value with the result of computation involving the previous value and arg. The operation is read-modify-write operation.

- operator+= performs atomic addition. Equivalent to return fetch_add(arg) + arg;.
- operator-= performs atomic subtraction. Equivalent to return fetch_sub(arg) - arg;.

1-4) 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)

5-8) The result may be an undefined address, but the operations otherwise have 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 any volatile overload participates in overload resolution. | (since C++20)

### 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 immediately preceding the effects of the corresponding member function in the modification order of *this).

### Notes

Unlike most compound assignment operators, the compound assignment operators for atomic types do not return a reference to their left-hand arguments. They return a copy of the stored value instead.

### 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

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

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