Question

I am trying to overload the subscript operator ([]) on an abstract class, the function called by the overload is implemented in the concrete object.

class CollectionBase {
public:
    double& operator[] (const int nIndex)
    {
        return getValue(nIndex);
    }

    virtual double getValue(int index) = 0;
};

class Collection    : public CollectionBase
{
    double getValue(int index) { return 0; }
};

The problem I am having is that my compiler is throwing an error on the call to getValue in the overload.

Initial value of reference to non-const must be an lvalue

Does anybody know the syntax for what I am trying to do?

Answer 1

The problem is that you are returning a reference to a temporary value returned from getValue. Either make both functions return double&, or both return double.