Question

When a function takes a parameter as a reference to a const object, I understand that the object passed as argument to it cannot be modified using the reference? So is there any scenarios in C++ where a const object can be modified through a reference to it? If yes, show an example.

Answer 1

C++ has a feature called mutable where a data member may be changed even through a const reference:

class Foo {
public:
    int a;
    mutable int b;
};

int main() {
    Foo f;
    f.a = 1; // ok
    f.b = 2; // ok
    const Foo &g = f;
    g.a = 1; // compile error
    g.b = 2; // ok
}

I get the following error:

In function 'int main()':
Line 12: error: assignment of data-member 'Foo::a' in read-only structure
compilation terminated due to -Wfatal-errors.

However, the assignment g.b = 2; succeeds.

This feature is usually used for private member variables only, where the changing of the data member does not affect the outside visible const-ness of the object. For example, it could be used as an optimisation to provide a way to cache previously calculated values.