NAME¶

std::atomic::operator+=,-= - std::atomic::operator+=,-=

Synopsis¶

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 (since C++20)
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.

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 (since C++20)
any volatile overload participates in overload resolution.

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 made ill-formed
cv-qualified) void or function

See also¶

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