const and inout Member Functions

const member functions

Some member functions do not make any modifications to the object that they are called on. An example of such a function is toString():

struct TimeOfDay { 
// ...
    string toString() {
        return format("%02s:%02s", hour, minute); 
    }
// ...
}

Since the whole purpose of toString() is to represent the object in a string format anyway, it should not modify the object.

A member function that does not modify the object is declared by the const keyword after the parameter list:

struct TimeOfDay { 
// ...
    string toString() const {
        return format("%02s:%02s", hour, minute); 
    }
}

That const guarantees that the object itself is not going to be modified by the member function. As a consequence, the toString() member function is allowed to be called even on immutable objects. Otherwise, the struct’s toString() could not be called: