Question

I have a problem with the following piece of code (it is a very simplified example that reproduce the error in my program) :

#include <iostream>

using namespace std;

template<class T> class CBase
{
    public:
        template <class T2> CBase(const T2 &x) : _var(x) {;}
        template <class T2> CBase (const CBase<T2> &x) {_var = x.var();}
        ~CBase() {;}
        T var() const {return _var;}
    protected:
        T _var;
};

template<class T> class CDerived : public CBase<T>
{
    public:
        template <class T2> CDerived(const T2 &x) : CBase<T>(x) {;}
        template <class T2> CDerived (const CBase<T2> &x) : CBase<T>(x) {;}
        ~CDerived() {;}
};

int main()
{
    CBase<double> bd(3);
    CBase<int> bi(bd); // <- No problem
    CDerived<double> dd1(3);
    CDerived<double> dd2(dd1);
    CDerived<int> di(dd1); // <- The problem is here
    return 0;
}

And the error is the following :

error: cannot convert 'const CDerived<double>' to 'int' in initialization

How to solve that ? (with a preference for modifications in the base class and not in the derived class, and if possible no use of virtuality)

Thank you very much

EDIT :
If I replace the concerned line with : CDerived<int> di(CBase<int>(CBase<double>(dd1))); it works but it is not very practical…

EDIT : Seems to be solved by that :

template <class T2> CDerived(const CDerived<T2> &x) : CBase<T>(static_cast<const CBase<T2>&>(x)) {;}

Answer 1

CDerived<int> di(dd1); // <- The problem is here

This invokes the first constructor of CDerived, and so T2 is inferred as CDerived<double> which is the type of dd1. Then, dd1 becomes x in the constructor; x which is CDerived<double>, gets passed to the base class constructor which accepts int (which is the value of the type argument T to CDerived class template). Hence the error, as CDerived<double> cannot be converted into int. Note that T of CBase is int.

See it as:

CDerived<int> di(dd1); // <- The problem is here
          ^       ^
          |       |
          |       this helps compiler to deduce T2 as double
          |
          this is T of the CDerived as well as of CBase

If you want to make your code work, then do this:

1. First derive publicly instead of privately.
2. Add another constructor taking CDerived<T2> as parameter.

So you need to so this:

template<class T> class CDerived : public CBase<T>  //derived publicly
{
    public:
        template <class T2> CDerived(const T2 &x) : CBase<T>(x) {;}

        //add this constructor
        template <class T2> CDerived(const CDerived<T2> &x) : CBase<T>(x.var()) {;}

        template <class T2> CDerived (const CBase<T2> &x) : CBase<T>(x) {;}
        ~CDerived() {;}
};

It should work now : online demo