error: passing 'xxx' as 'this' argument discards qualifiers

Question

Can anyone tell me why the following code:

#include <iostream>
using namespace std;

class Test {
    int a, b, c;
public:
    Test() : a(1), b(2), c(3) {}
    const void print() {cout << a << b << c;}
    int sum() {return (a+b+c);}
};

const Test& f(const Test& test) {
    test.print();
    // cout << test.sum();
    return test;
}

main() {
    Test x;
    cout << "2: ";
    y = f(x);
    cout << endl;
}

gives the compile error

"error: passing 'const Test' as 'this' argument discards qualifiers"

?

My print() method is const, which is all I'd understood was necessary. For me the (commented out) sum() method in f() should give an error, but not the print() method. If anyone can point out to me where I am misunderstanding - that would be great.

Answer 1

You're calling a non-const method print() on a const object. A const method is such that cannot modify the object it's called on, and this is the only sort of member methods you are allowed to call on const objects (to preserve the const-ness). A const method is denoted by const after the method arguments list:

void Test::print() const {
   cout << a << b << c;
}

And yes, const void is useless at best, just void is all the same.