member only of `atomic<` (C++11) and `atomic<` (C++20) template specializations | ||

(1) | ||

T fetch_add( T arg, std::memory_order order = std::memory_order_seq_cst ) noexcept; | ||

T fetch_add( T arg, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; | ||

member only of `atomic<T*>` template specialization | ||

(2) | ||

T* fetch_add( std::ptrdiff_t arg, std::memory_order order = std::memory_order_seq_cst ) noexcept; | ||

T* fetch_add( std::ptrdiff_t arg, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; |

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 read-modify-write operation. Memory is affected according to the value of `order`

.

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 operation otherwise has 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 conform to the corresponding The volatile-qualified versions are deprecated if | (since C++20) |

arg | - | the other argument of arithmetic addition |

order | - | memory order constraints to enforce |

The value immediately preceding the effects of this function in the modification order of `*this`

.

#include <iostream> #include <thread> #include <atomic> #include <array> 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

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 |

(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) |

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