How can I simulate interfaces in C++?

Question

Since C++ lacks the interface feature of Java and C#, what is the preferred way to simulate interfaces in C++ classes? My guess would be multiple inheritance o

Answer 1

There is no good way to implement an interface the way you're asking. The problem with an approach such as as completely abstract ISerializable base class lies in the way that C++ implements multiple inheritance. Consider the following:

class Base
{
};
class ISerializable
{
  public:
    virtual string toSerial() = 0;
    virtual void fromSerial(const string& s) = 0;
};

class Subclass : public Base, public ISerializable
{
};

void someFunc(fstream& out, const ISerializable& o)
{
    out << o.toSerial();
}

Clearly the intent is for the function toSerial() to serialize all of the members of Subclass including those that it inherits from Base class. The problem is that there is no path from ISerializable to Base. You can see this graphically if you execute the following:

void fn(Base& b)
{
    cout << (void*)&b << endl;
}
void fn(ISerializable& i)
{
    cout << (void*)&i << endl;
}

void someFunc(Subclass& s)
{
    fn(s);
    fn(s);
}

The value output by the first call is not the same as the value output by the second call. Even though a reference to s is passed in both cases, the compiler adjusts the address passed to match the proper base class type.