malloc/free and new/delete compatibility in C++?

Question

There is a nice comparison of malloc/free and new/delete here, and good explanations how malloc() and free() work here. Obviously, we shall not mix them - use free with new or delete with malloc.

We can see a lot of open source projects, with many contributors, using both of these mechanisms, while respecting the above "no-mix" rule. Usualy, you have only one way in one file (one author, one preference). I have forked such a project and I am adding some functionalities with use of new/delete. But I encounter some curious memory corruptions. Of course, I am probably responsible of them, but.....

That leads me to ask some "naive" questions :

Can I have both mechanisms malloc/free and new/delete in the same compilation unit (*.o) - of course, respecting the "no-mix" rule ?

Can I interleave the two mechanisms, like in this code ?

int *a = (int *) malloc (1000 * sizeof int);

int *b = new int[1000];

// some code

free a;

delete[] b;

Yes but you shouldn't even be using `new` or `malloc` in your own code; use smart pointers like `unique_ptr` or `shared_ptr` instead. — Simple, Oct 03 '13 at 09:01
Do you ensure not to ever call `malloc()` and `free()` for a type having a constructor or destructor? These calls would not execute the constructor and destructor of your class and may thus be a cause of problems. — Julien, Oct 03 '13 at 09:10

score 7 · Accepted Answer · edited May 23 '17 at 11:53

7

Yes, you can interleave them - just use a deallocation function matching the one used for allocation. The problem only arises if you use a wrong deallocation function.

Note that it's not that easy when you have several libraries - those might use different heaps and so even though you use a right named function it might happen to be implemented in the wrong module and so use the wrong heap which will drive you right into undefined behavior. See this answer for a better idea of what I'm talking about here.

edited May 23 '17 at 11:53

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answered Oct 03 '13 at 09:03

sharptooth

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And bear in mind that `malloc` can only be used for POD data types, because it doesn't run constructors. – Angew is no longer proud of SO Oct 03 '13 at 09:08
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@Angew you can still use `malloc` you just have to use placement `new` and call the destructor yourself. `T* p = ::new(std::malloc(sizeof(T))) T(some, args); p->~T(); std::free(p);` – Simple Oct 03 '13 at 09:10
@Simple Well, yes. But then the question is why. – Angew is no longer proud of SO Oct 03 '13 at 09:12
@Simple: You've omitted the `malloc()` return value null check. – sharptooth Oct 03 '13 at 09:12
@Angew you're right you wouldn't write it if you have another choice but sometimes a C library will `malloc` the memory for you and you have to do it this way. – Simple Oct 03 '13 at 09:16

malloc/free and new/delete compatibility in C++?

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