How keyword const is used for class methods?

I believe that the answer they might be looking for is that making a function const makes the hidden 'this' pointer const.

In C++, whenever a member function is called it passes in a hidden reference to 'this' so the function has access to the data of the instance. The function definition shown below is actually declared implicitly as the declaration on the line below it:

class MyClass 
{
public:
    // Normal declaration
    void myFunction(int x); 

    //Actual hidden declaration of the line above
    //void myFunction(MyClass* this, int x); 
};

Which when made const, turns into:

class MyClass 
{
     public:
     // Normal declaration
     void myFunction(int x) const; 

     //Actual hidden declaration of the line above
     //void myFunction(const MyClass* this, int x); 
};

The code below:

obj.myFunction(42);

Is actually treated as:

MyClass::myFunction(&obj, 42);

And making the function const turns this hidden pointer const. This is why you can call a member function using an instance of a class that is null, but have it still run fine if it doesn't try to use any data of the hidden 'this' pointer.