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

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< cpp‎ | atomic‎ | atomic

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

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

1) Performs atomic addition. Equivalent to fetch_add(arg) + arg.
2) Performs atomic subtraction. Equivalent to fetch_sub(arg) - arg.
3) Performs atomic bitwise and. Equivalent to fetch_and(arg) & arg.
4) Performs atomic bitwise or. Equivalent to fetch_or(arg) | arg.
5) Performs atomic bitwise exclusive or. Equivalent to fetch_xor(arg) ^ arg.

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 operations otherwise have 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 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 cv void or function | made ill-formed

### See also

operator++operator++(int)operator--operator--(int) | increments or decrements the atomic value by one (public member function)

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