Malloc vs calloc | memory not initialized

Question

Hi

I have a sample program where I tried to verify that memory is initialized with calloc and not with malloc?

Why output is 0 for both cases ? Should not malloc initialize value to 22?
https://sololearn.com/compiler-playground/cbq89p8w5AtY/?ref=app

Answer 1

Evidently, it is merely casting the pointer types, not instantiating the objects. The statement a->display() works anyway because the display() method is available in memory regardless of whether the object is instantiated or not. Its address is resolved at compile time. The value of instance variable, a, is found at whatever offset from the pointer that it normally would be found, as though it were instantiated. Effectively you have created memory allocation for the objects but bypassed the constructor.

Answer 2

Thanks Brian for details.

I got the point now that new operator is the one which calls constructors , not the c related memory allocation functions.

However, How to verify that malloc is less costlier compared to calloc as calloc has to initialize also. I thought I will have cout in constructor , but that does not get called at all.

Answer 3

If you don't trust the literature that tells you calloc takes longer, then you could try timing tests using time or perf. Or examine the assembly output from the compiler (look up the switch to use for your compiler).

However, isn't it just common sense that after calloc performs the OS API call to allocate memory it then has to do an extra step to clear the memory?