Counting With Template Metaprogramming?

Question

I've been trying to think up a creative solution to this problem (on and off) for some time, but I have not as of yet been able to. I recently considered that it might be solvable with template metaprogramming, though I am not sure due to my relative lack of experience with the technique.

Is it possible to use template metaprogramming (or any other mechanism with the C++ language) to count the number of classes which are derived from some base class such that each derived class is given a unique, static class identifier?

Thanks in advance!

Answer 1

No. This is a problem that comes up in practice quite a lot, and as far as I'm aware there are only two solutions:

1. Manually assign IDs to each derived class.
2. Dynamically and lazily generate IDs non-deterministically.

The way you do the second one is something like this:

class Base
{
    virtual int getId() const = 0;
};

// Returns 0, 1, 2 etc. on each successive call.
static int makeUniqueId()
{
    static int id = 0;
    return id++;
}

template <typename Derived>
class BaseWithId : public Base
{
    static int getStaticId()
    {
        static int id = makeUniqueId();
        return id;
    }

    int getId() const { return getStaticId(); }
};

class Derived1 : public BaseWithId<Derived1> { ... };
class Derived2 : public BaseWithId<Derived2> { ... };
class Derived3 : public BaseWithId<Derived3> { ... };

This gives you unique IDs for each class:

Derived1::getStaticId(); // 0
Derived2::getStaticId(); // 1
Derived3::getStaticId(); // 2

However, those IDs are assigned lazily, so the order you call getId() affects the ID's returned.

Derived3::getStaticId(); // 0
Derived2::getStaticId(); // 1
Derived1::getStaticId(); // 2

Whether or not this is OK for your application depends on your particular needs (e.g. would be no good for serialisation).

Answer 2

Is it possible to use template metaprogramming (or any other mechanism with the C++ language) to count the number of classes which are derived from some base class such that each derived class is given a unique, static class identifier?

No, there's no such mechanism. No matter what you do, you'll have to add "something" (most likely a macro) to every derived class manually to achieve something like that. See Qt 4 and Q_OBJECT macro. You could also make a macros for making derived classes, but this can't be done automatically.

You could however write your own C++ code preprocessor/analysis tool that scans source code you provided and then inserts necessary directives into source.

Also, RTTI provides name for every class. The problem is that this name is implementation-specific, so it isn't very useful.