Question

I have a problem with whether making class members constant or not. Let me give an example.

#include <iostream>
class ValueClass
{
    int ival;
public:
    void set(int i) {ival = i;}
    int get() {return ival;}
};

class XXX;

class ABC
{
    ValueClass vc;  
public:
    int getval() const {return vc.get() ;}  
friend class XXX;
};

class XXX
{
    std::vector<ABC> abclist;
    void Invalidate()
    {
    // Iterates through abclist and modifies ValueClass members.
    // e.g. abclist[i].vc.set(i);
    }
};

class QWE
{
    const ABC & abc; 
public:
    QWE(const ABC & abc_): abc(abc_) { }
    const ABC & getABC() { return abc; } 
};

int main()
{
    ABC abc;
    QWE qwe(abc);

    std::cout << qwe.getABC().getval()  << "\n"; // Compiler error
}

My ABC class holds an instance of ValueClass which is responsible for having getter and setter for an int value. Also QWE class has an ABC member and I need to obtain this member. I am told that returning the abc as non-const is a very bad practice. But here comes the problem, I can not use non-const vc in a const int getval() function and I simply can not make it const because in another thread XXX::Invalidate() is called. This function changes the data in ValueClass with respect to some incoming data.
Obviously there is something wrong with my design, I can not blame the C++ language. How can I solve this problem?

Answer 1

Make get() const in ValueClass

  class ValueClass
  {
      int ival;
    public:
      void set(int i) {ival = i;}
      int get() const {return ival;} // make get const
  };

Probably should do the same for getABC

  const ABC & getABC() const { return abc; } 

This means that this method does not change the object it is called on.