Issue regarding malloc and free in C++

[mayank_3103]

Hi All,
I am having a confusion in my mind (it is more or less conceptual) that if I am allocating a memory to any object by using "new". and later deallocating that by using "free" what problem will I be facing? Or will it be running smoothly?

Please help me regarding to this problem.

Thanks.

[VictorN]

The rule is very simple:
all that was newed must be deleted and
all what was mallocked must be freed.
The result of any mixture of those - the undefined behavior and a lot of various problems (it might be "access violation" as well as many other errors, and some of them may appear in some other places than the problem lies)

[monarch_dodra]

The problem you will be facing is:
- Objects that are deallocated but not destroyed: free does not destroy objects (bad).
- Undefined behaviour: It is illegal to free a new, or delete a malloc (extremely bad).

Under no circumstances is it OK to free a new. You should not even "try to see if it runs".

[mayank_3103]

Thanks for the reply and I already applied in my demo code, and it worked. Though It might not destroyed the object, but It didnt shown me any error.

So there must be any specific reason to avoid this type of programming, or its just the rule to use delete with new and free with malloc?

Somewhere I read not to mix C and C++ concepts together as malloc is in C and new is given by C++, but I dont think this is the main reason.

Any help will be greatly appreciated. This doubt is not yet cleared with me.

Thanks.

[monarch_dodra]

malloc simply allocates from the heap. new is a C++ keyword, that calls "operator new", which in turn can be overloaded to do weird and wonderful things behind the scene automatically: stack allocation; Pool allocation; logging.. You name it!

Thus malloc/free are not compatible with new/delete.

...

This is a simple explanation, but you should not rely on it. In particular, don't think you can delete a new just because your new is not overloaded. The bottom line is that mixing them has been declared ILLEGAL by the standard, and doing it creates undefined behaviour. That means you should not try to understand, nor test it. You don't do it, period.

The thing about undefined behaviour is that the behaviour is undefined. You can test it as much as you want, and your results will be irrelevant.

Just because it (seems to) works doesn't mean it is right. That is both the beauty, and the bane, of C++.

[ninja9578]

I'm also fairly certain that new will word align your objects, so it might actually create buffers on either side that you don't know about. In the end, new actually uses malloc, but it does other stuff too, and like monarch said, it can be overloaded.

You can actually do your allocation of objects with malloc/free if you really want to:

Object * obj = new((Object*)malloc(sizeof(Object))) Object();

obj -> ~Object();
free(obj);

But that's only really used for advanced memory management like memory pools and such. It can be significantly faster, but not always.

[Paul McKenzie]

Thanks for the reply and I already applied in my demo code, and it worked. Though It might not destroyed the object, but It didnt shown me any error.

There are many things in C++ that will not show you an error, even though it's wrong. It is not guaranteed that C++ will tell you that what you're doing is incorrect -- you must know that for yourself, and not rely on the running of a program to tell you this.

So there must be any specific reason to avoid this type of programming, or its just the rule to use delete with new and free with malloc?

delete calls the destrucctor oif the object, free() does not. If you've written a user-defined destructor, that destructor will not get the chance to run, and you can have all sorts of things wrong happening.

Somewhere I read not to mix C and C++ concepts together as malloc is in C and new is given by C++, but I dont think this is the main reason.

The main reason is that new and malloc are not the same thing. The former creates objects dynamically, the latter does not. All malloc() does is give you a bunch of bytes. Nothing more than that. It doesn't give you an object properly constructed.

Regards,

Paul McKenzie