std::atomic<T>:: fetch_add
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| Member functions | ||||
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(C++20)
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(C++20)
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(C++20)
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| Constants | ||||
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(C++17)
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| Specialized member functions | ||||
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Specialized for integral,
floating-point (C++20) and pointer types |
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atomic::fetch_add
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Specialized for integral and
pointer types only |
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(C++26)
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(C++26)
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| Specialized for integral types only | ||||
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member only of
atomic<
Integral
>
specializations
and
atomic<
Floating
>
specializations
(since C++20)
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T fetch_add
(
T arg,
std::
memory_order
order
=
std:: memory_order_seq_cst ) noexcept ; |
(1) | (since C++11) |
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T fetch_add
(
T arg,
std::
memory_order
order
=
std:: memory_order_seq_cst ) volatile noexcept ; |
(2) | (since C++11) |
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member only of
atomic<T*>
partial specialization
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T
*
fetch_add
(
std::
ptrdiff_t
arg,
std::
memory_order
order
=
|
(3) | (since C++11) |
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T
*
fetch_add
(
std::
ptrdiff_t
arg,
std::
memory_order
order
=
|
(4) | (since C++11) |
Atomically replaces the current value with the result of arithmetic addition of the value and arg . That is, it performs atomic post-increment. The operation is a read-modify-write operation. Memory is affected according to the value of order .
|
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 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) |
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 overload (2) or (4) participates in overload resolution. |
(since C++20) |
Parameters
| arg | - | the other argument of arithmetic addition |
| order | - | memory order constraints to enforce |
Return value
The value immediately preceding the effects of this function in the modification order of * this .
Example
#include <array> #include <atomic> #include <iostream> #include <thread> std::atomic<long long> data{10}; std::array<long long, 5> return_values{}; void do_work(int thread_num) { long long val = data.fetch_add(1, std::memory_order_relaxed); return_values[thread_num] = val; } int main() { { std::jthread th0{do_work, 0}; std::jthread th1{do_work, 1}; std::jthread th2{do_work, 2}; std::jthread th3{do_work, 3}; std::jthread th4{do_work, 4}; } std::cout << "Result : " << data << '\n'; for (long long val : return_values) std::cout << "Seen return value : " << val << std::endl; }
Possible output:
Result : 15 Seen return value : 11 Seen return value : 10 Seen return value : 14 Seen return value : 12 Seen return value : 13
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
|
(C++11)
(C++11)
|
adds a non-atomic value to an atomic object and obtains the previous value of the atomic
(function template) |
|
increments or decrements the atomic value by one
(public member function) |