# std::free

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< cpp‎ | memory‎ | c

Defined in header `<cstdlib>` |  |
void free( void* ptr ); |  |

Deallocates the space previously allocated by std::malloc(), std::calloc() or std::realloc().

If `ptr` is a null pointer, the function does nothing.

The behavior is undefined if the value of `ptr` does not equal a value returned earlier by std::malloc(), std::calloc(), or std::realloc().

The behavior is undefined if the memory area referred to by `ptr` has already been deallocated, that is, `std::free()` or std::realloc() has already been called with `ptr` as the argument and no calls to std::malloc(), std::calloc() or std::realloc() resulted in a pointer equal to `ptr` afterwards.

The behavior is undefined if after `std::free()` returns, an access is made through the pointer `ptr` (unless another allocation function happened to result in a pointer value equal to `ptr`)

The following functions are required to be thread-safe:

- The library versions of `operator new` and `operator delete`
- User replacement versions of global `operator new` and `operator delete`
- std::calloc, std::malloc, std::realloc, std::aligned_alloc (since C++17)

Calls to these functions that allocate or deallocate a particular unit of storage occur in a single total order, and each such deallocation call happens-before the next allocation (if any) in this order.

(since C++11)

### Parameters

ptr | - | pointer to the memory to deallocate

### Return value

(none)

### Notes

The function accepts (and does nothing with) the null pointer to reduce the amount of special-casing. Whether allocation succeeds or not, the pointer returned by an allocation function can be passed to `free()`

### Example

Run this code

    #include <cstdlib>

    int main()
    {
        int* p1 = (int*)std::malloc(10*sizeof *p1);
        std::free(p1); // every allocated pointer must be freed

        int* p2 = (int*)std::calloc(10, sizeof *p2);
        int* p3 = (int*)std::realloc(p2, 1000*sizeof *p3);
        if(p3) // p3 not null means p2 was freed by std::realloc
           std::free(p3);
        else // p3 null means p2 was not freed
           std::free(p2);
    }

### See also

C documentation for free

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