Question

Why would the compiler complain at the indicated line?

class C
{
    std::string s;
public:
    C() { s = "<not set>";}
    ~C() {}
    void Set(const std::string ss) { s=ss; }
    const std::string Get() { return s; }

    C &operator=(const C &c) { Set(c.Get()); return *this; }
    //error: passing ‘const C’ as ‘this’ argument of ‘const string C::Get()’
    // discards qualifiers [-fpermissive]


    //C &operator=(C &c) { Set(c.Get()); return *this; }   <-- works fine

};

Answer 1

You need to declare the function Get() to be const:

const std::string Get() const { return s; }

Even though Get() doesn’t change any member values, the compiler is instructed to only let you call functions that are explicitly labeled const.

gcc is instructing you that you can override it’s complaint by using the argument -fpermissive; however, it’s generally better not to do this (or else why declare anything const at all?). Generally, it’s better to make sure every member function called on a const parameter is a const member function.

This article concerning Const Correctness is very interesting.