Thread: problem: passing `xx' as `this' argument of `int xx::accessor()' discards qualifiers

Hi, I wrote a simplified version of a program I was having a problem with, to better illustrate my questions:
1. How do I bypass this error?
2. What is the exact cause?

Code:

#include <iostream>

using namespace std;

class Stuff {
        public:
                Stuff();
                Stuff(int);
                Stuff(const Stuff&);
                int accessor();
                void mutator(int);
        private:
                int mystery;
};

Stuff::Stuff(): mystery(0) {}
Stuff::Stuff(int b): mystery(b) {}
Stuff::Stuff(const Stuff& b)
{
        mystery = b.accessor();
}
int Stuff::accessor() { return mystery; }
void Stuff:: mutator(int b) { mystery = b; }

int main()
{
        Stuff fool;
        cout << fool.accessor() << endl;
        return 0;
}

and the error that spits out:

Code:

bash-3.00$ !g
g++ test.c
test.c: In copy constructor `Stuff::Stuff(const Stuff&)':
test.c:20: error: passing `const Stuff' as `this' argument of `int Stuff::accessor()' discards qualifiers
bash-3.00$

Thanks dudes.

The accessor function should be const, since it does not modify the state of the instance it is called on.

Of course, there's no real need for you to write your own copy constructor, the compiler generated one will work correctly. Even if you did (because the actual code is different), I would use the initializer list and access mystery directly.

Regardless, you should make your accessor function const correct to fix the error.