Question

Let’s say I have a

class A
{
  int A1;
  void Af();
};

Then I do:

A  hA;
A* pA = new A();

now, hA is an object in the stack; I can use

hA.A1 = 52 // for example

but pA just points to class A, how is that useful or handy? (any examples please?)

second issue:

when I did A* pA = new A(); did I allocate anything in the heap? was there any malloc() in the background? if not, then why not? and how come int *p_array = new int[5] will allocate memory in the heap of 5 ints and not A* pA = new A()?

Answer 1

A  hA = new A();

leads to a compiler error. The correct way of defining a variable “on the stack” is

A ha;

but pA just points to class A, how is that useful or handful (any examples please?)

In some place in the memory, you have an object of type A, and pa points to it. If you want to set the A1 member of that object to 52, you write

pA->A1 = 52;

The why is it useful part is not a real question.

when I did A* pA = new A(); did I allocate anything in the heap?

Yes, you did. new does two things: it allocates memory and invokes the constructor.

was there any malloc() in the background?

That is unspecified, but in many implementations new is implemented via malloc

and how come int[] A = A[42] will allocate memory in the heap of 42 ints and not A pA = new A() ?

This, sir, is also a compiler error. What you meant was

int* A = new A[42];

This is the operator new[] which allocates arrays on the heap and calls constructors if necessary (in case of ints it isn’t).