Question

I have two questions.

1) What does a constructor really do? What happens if we don’t use constructors while declaring an instance?

2) Can you tell me the difference between these two?

A a(1,2)

A *a = new A(1,2)

Best regards.

Answer 1

A constructor initializes the member variables of the class so that it is ready for use. The result of not using a constructor when declaring an instance varies on context.

If you are allocating a variable on the heap, like so:

A *a;

a will point to a random address in memory until it is assigned to NULL or 0, or an existing or new instance of a class, e.g.:

A *a = new A(1, 2);

If you are allocating a variable on the stack, the following syntax is used:

A a(1, 2); // if parameters are used
A a; // if no parameters are used

Both of the above call the constructor of class A, allocating an instance of it on the stack. So, that answers both your questions – your first example allocates an instance of A on the stack, and the second allocates an instance of A on the heap.