which function is better calloc or malloc ?

It depends on what you want. calloc(...) additionally sets the memory to 0. See:

Calloc also allows to specify the number of items and the size of each item. With malloc you have to explicitly do the calculation.

Is this for c or C++? In C++ don't use these. Use new/delete.
For C++? Neither, you should be using new.

For C? Probably calloc since it initializes memory to 0 which is free or next to free on modern systems. This allows you to design data structures such that a zero-initialized struct is an invariant and "uninitialized" structs allocated on the heap are not possible. This is not necessary in C++ since we have the new operator and construtors, but that can be useful in C.
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