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std::free - cppreference.com

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).

(none)

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.

#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 null means realloc failed and p2 must be freed.
        std::free(p2);
    std::free(p3); // p3 can be freed whether or not it is null.
}

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