# std::atomic<T>::fetch_sub

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

| (1) | |

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

T fetch_sub( T arg,
std::memory_order order = std::memory_order_seq_cst ) volatile noexcept;
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member only of `atomic<T*>` template specialization | | |

| (2) | |

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

T* fetch_sub( 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 subtraction of the value and `arg`

. That is, it performs atomic post-decrement. 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 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 other argument of arithmetic subtraction |

order | - | memory order constraints to enforce |

### Return value

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

.

### 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

(C++11)(C++11) | subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic (function template) |

| increments or decrements the atomic value by one (public member function) |