What if, memory allocated using malloc is deleted using delete rather than free

Question

I came across an issue which I could not resolve.

My question is, if I used malloc to allocate memory and then memory block is delete using delete

Answer 1

No, it is not a rule of thumb that you should not mix new/free or malloc/delete.

Sure, it might be possible to use free to relinquish an allocation you obtained with new without a crash, but it is highly implementation/system specific.

Ignoring the fact that you're bypassing destructors or writing code that will corrupt memory on some systems, the more fundamental, more basic C++ consideration is really simply this:

malloc/free are global, system functions, while new and delete are operators.

struct X {
    void* operator new(size_t);
    void operator delete(void* object);
};

Sure - you can borrow a book from Amazon and return it to your local library: go there late one night and leave it on their front door step.

It's going to have undefined behavior, up to and including your getting arrested - and I'm talking about mixing new/free again here :)

Put another way: malloc and free deal with raw memory allocations, while new and delete deal with objects, and that's a very different thing.

void* p = new std::vector(100);
void* q = malloc(sizeof(*q));