< cpp‎ | memory‎ | c
Utilities library
General utilities
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Integer comparison functions
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Common vocabulary types

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(until C++17)
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C Library
Low level memory management
Defined in header <cstdlib>
void free( void* ptr );

Deallocates the space previously allocated by std::malloc, std::calloc, std::aligned_alloc (since C++17), 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, std::aligned_alloc (since C++17), 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, std::aligned_alloc (since C++17), 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:

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)


[edit] Parameters

ptr - pointer to the memory to deallocate

[edit] Return value


[edit] 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 std::free.

[edit] Example

#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
    else // p3 null means p2 was not freed

[edit] See also