Question

Is it possible to pass some parameters to constructor of a class inside a constructor of another class using malloc? I can do that with new. I need to do the same thing with malloc:
(If it does not make sense, consider that I am using a custom allocator instead of malloc)

Class A_Class ... {
       public:
       B_Class *b;
...
     A_Class: ...
      { b = new B_Class (c1, c2, c3);
      } // end of constructor
}

Now with malloc:

Class A_Class ... {
       public:
       B_Class *b;
...
     A_Class: ...
      { b = (B_Class*) malloc (sizeof(*b)); 
        ??????????????? }
}

Answer 1

malloc allocates raw memory. There’s no point in trying to pass constructor arguments to it because it doesn’t call any constructors.

If you have to work with raw memory, it is up to you to construct an object in previously allocated raw memory by using “placement new” syntax

...
void *raw_b = malloc(sizeof *b);
b = new(raw_b) B_Class(c1, c2, c3); // <- placement new 
...

Numerically, the value of b will be the same as raw_b, i.e. it is possible to get by without an extra raw_b pointer. But I prefer to do it this way, with an intermediate void * pointer, to avoid ugly casts.

Be careful when destroying such objects, of course. Custom allocation requires custom deletion. You can’t just delete your b in general case. A symmetrical custom deletion sequence would involve an explicit destructor call followed by raw memory deallocation

b->~B_Class(); // <- explicit destructor call
free(b);

P.S. You might consider going a different route: overloading operator new/operator delete in your class instead of spelling out custom allocation explicitly every time you need to create an object.