Question

Can anyone tell me how you can make sure a method can only access and not change private data members in a C++ class instance is the method does not have any arguments. I am writing a program that draws shapes, and I tried this

void drawCircle() const;

but I have been unable to determine whether the data members remain unchanged. Thanks

Answer 1

Declaring a member-function const (like your example) means that it won’t change any member variables via (the implicit) this pointer. The compiler enforces this.

So the following won’t compile:

class Foo
{
public:
    void drawCircle() const
    {
        x = 5;  // equivalent to this->x = 5; and this is const here
    }
private:
    int x;
};

Of course, it doesn’t prevent you playing tricks like casting away the const-ness.