delete/free memory allocated by malloc and reused by new

Question

In the following program new reuses a memory allocated by malloc. But how to free the memory then? by free or by delete? how to call the destructor?

#include <iostream>

struct A
{
    A()  {}
    ~A() {}
};
int main()
{
    void* p = malloc(sizeof(A));
    A* pa = new (p) A();

    // codes...


    delete pa;
    // pa ->~A();
    // free(p);
}

Is it safe to reuse malloc memory by new? And how to free the memory?

@πάνταῥεῖ a class may need a call to destructor. how to call the destructor? explicitly before freeing? or after freeing? — Minimus Heximus, Mar 07 '19 at 12:03

Maxim Egorushkin · Accepted Answer · 2019-03-07T12:06:14.607

1

new (p) A() is placement new that doesn't allocate memory and only calls the constructor.

Calling regular delete on that pointer returned by placement new is undefined behaviour.

To deallocate correctly here you need to call the destructor and then free the memory:

pa->~A(); // or, in C++17 std::destroy_at(pa);
free(pa);

edited Mar 07 '19 at 12:06

answered Mar 07 '19 at 12:01

Maxim Egorushkin

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can `destroy_at` be used anywhere one needs to call the destructor explicitly? – Minimus Heximus Mar 07 '19 at 12:09
1

@nano I do not see why not. Unless you want to avoid `#include `. – Maxim Egorushkin Mar 07 '19 at 12:16

delete/free memory allocated by malloc and reused by new

1 Answers1