Question

I understand when using it as follows:

int getX() const {...} 

means that this function will not modify any of the variables used in its body. But what I didnt undertsnad is using const in 2 places, like the following:

const int* getX () const {...}

what’s the use of putting the const keyword before the int*?

Answer 1

Your first interpretation is wrong.

int getX() const { ... }

is a member function, and it cannot modify any data members or call any non-const for a given instance of the class that has that function.

const int* getX() const { ... }

returns a const int*, so it limits what you can assign to using it. It is a non-const pointer to const int, so you cannot modify the int it points to, but you can modify the pointer itself. For example:

const int* i = someInstange.getX(); // getX() returns const int*
i = someOtherFunction(); // another function returning const int*. OK to reassign i.

So i itself isn’t const, but what it points to is:

(*i)++; // ERROR!

if you wanted to return a const pointer to const int you would need something like this:

const int * const getX() {}

The whole issue is further complicated because you can put the first const in different places without changing the meaning. For more information on that, look at this SO question.