What happens to dynamic memory when C and C++…

When interacting between C and C++ programs that use dynamic memory allocation/deallocation (using malloc/free from C and new/delete from C++), issues can arise due to differences in memory management and constructor/destructor calls.

Main issues:

Different memory allocators: C and C++ may use different implementations of memory allocators, leading to errors when trying to free memory allocated by one with the other.
Constructor and destructor calls: new and delete in C++ not only allocate/deallocate memory but also call constructors and destructors of objects. malloc and free in C do not do this. If C++ code allocates memory for an object, but C code frees it using free, the destructor in C++ will not be called, which can cause resource leaks or incorrect state. Similarly, if C code allocates memory, and C++ code tries to free it with delete, errors may occur because delete expects to call the object's destructor.

To prevent such issues, the following rules should be followed:

If memory is allocated with malloc in C, it should be freed with free in C.
If memory is allocated with new in C++, it should be freed with delete or delete[] in C++.
If a library or module provides functions for allocating and freeing memory, those should be used regardless of whether the calling code is in C or C++.

Example of incorrect interaction:

// C++ code
#include <iostream>
#include <cstdlib> // For malloc and free

class MyClass {
public:
    int data;
    MyClass(int d) : data(d) { std::cout << "Constructor: " << data << std::endl; }
    ~MyClass() { std::cout << "Destructor: " << data << std::endl; }
};

extern "C" {
    void process_memory(void* ptr);
}

int main() {
    MyClass* obj = new MyClass(10); // Allocation in C++
    process_memory(obj); // Passing to C
    return 0;
}

// C code (file process_memory.c)
#include <stdlib.h>

void process_memory(void* ptr) {
    // ... use ptr ...
    free(ptr); // Freeing in C - incorrect for a C++ object
}

In this example, the destructor of MyClass will not be called when freeing memory in C code.

What happens to dynamic memory when C and C++ programs interact?