Question

I have a non-template abstract base class which is used in order to be able to have a reference to the base type, since unspecialized template types can’t be used as method arguments.

#ifndef RPCPP_ICALLBACKBASE_H
#define RPCPP_ICALLBACKBASE_H

#include <string>

namespace rpcpp
{
    class ICallbackBase
{
public:
    virtual ~ICallbackBase() {};
    virtual void OnSuccess(void result) = 0;
    virtual void OnError(std::string error) = 0;
};
}

#endif // RPCPP_ICALLBACKBASE_H

The abstract template class ICallback inherits from ICallback base, like so:

#ifndef RPCPP_ICALLBACK_H
#define RPCPP_ICALLBACK_H

#include "ICallbackBase.h"

namespace rpcpp
{
template <class T>
class ICallback : public ICallbackBase
{
public:
    virtual ~ICallback() {};
    virtual void OnSuccess(T result) = 0;
    virtual void OnError(std::string error) = 0;
};
}

#endif // RPCPP_ICALLBACK_H

Finally, one can create a concrete type by inheriting from ICallback:

#ifndef RPCPP_SAMPLE_CALLBACK_H
#define RPCPP_SAMPLE_CALLBACK_H

#include "ICallback.h"
#include <iostream>

namespace rpcpp
{
class SampleCallback : public ICallback<double>
{
public:
    ~SampleCallback() {};

    virtual void OnSuccess(double result)
    {
        std::cout << "Successfully executed a remote procedure, A + B = " << result << "\r\n\r\n";
    }

    virtual void OnError(std::string error)
    {
        std::cout << "Error while executing a remote procedure: " << error << "\r\n\r\n";
    }
};
}

#endif // RPCPP_SAMPLE_CALLBACK_H

All of which compiles nice, however when i try to use this, like so:

rpcpp::SampleCallback sc;
sic.CalculateMean(15, 28, &sc); // Third argument of this method is expected to be ICallbackBase&.

It produces the following two errors:

“cannot instantiate abstract class” in line #1.
“cannot convert parameter 3 from SampleCallback& to ICallbackBase&” in line #2

What am I doing wrong?

Answer 1

virtual void OnSuccess(void result) = 0;

is never defined.