Modifying reference member from const member function in C++

Question

I am working on const-correctness of my code and just wondered why this code compiles:

class X {     int x;     int& y; public:     X(int& _y):y(_y)     {     } void f(int& newY) const     {         //x = 3; would not work, that's fine         y = newY; //does compile. Why?     } };  int main(int argc, char **argv)  {     int i1=0, i2=0;     X myX(i1);     myX.f(i2); ... } 

As far as I understand, f() is changing the object myX, although it says to be const. How can I ensure my compiler complains when I do assign to y? (Visual C++ 2008)

Thank a lot!

Answer 1

Because you are not changing any variable in X. Actually, you are changing _y which is an outsider with respect to your class. Don't forget that:

y = newY; 

Is assigning the value of newY to the variable pointed by y, but not the references them selves. Only on initialization the references are considered.

Answer 2

The situation is similar to pointer members. In a const member function, the const applies to the pointer itself, not the pointee.

It's the difference between:

X* const //this is how the const applies: you can modify the pointee const X* 

Except X& const isn't valid syntax, since the reference can't be made to refer to another object in the first place (they are implicitly always const). In conclusion: const on methods has no effect on member references.