Question

I want to do:

template <class Derived=BattleData>
class BattleData : public BattleCommandManager<Derived> {
};

But obviously BattleData isn’t declared, so I tried a forward declaration:

template <class T> class BattleData;

template <class Derived=BattleData>
class BattleData : public BattleCommandManager<Derived> {
};

But then I get

error: “wrong number of template parameter on the second line, with
BattleData.

I really fail to see a solution to this!

Edit:

The reason I’m doing this is because I want to be able to use BattleData directly as a class, but I also want to be able to subclass it in which case I have to specify the derived class as the second template parameter.

For example let’s say the corpus of my BattleData class is :

template <class Derived> class BattleData: public BaseClass<Derived> {
    void foo1(){};
    void foo2(){};
    void foo3(){};
}

And I have a subclass

template class SubBattleData: public BattleData<SubBattleData> {
    void foo1(){};
}

I would still want, in some cases, to be able to write code like this:

BattleData *x = new BattleData(...);

I can’t even do the following without being able to use default arguments:

BattleData<BattleData> *x = new BattleData<BattleData>(...);

On one side, the reason functions aren’t virtualized in the BattleData class is the benefit of having no virtual function. The other reason it doesn’t work for me is that one of the parent CRTP classes invokes functions only if they’re present in the derived type (using decltype(Derived::function) and enable-if like structures), and fall back to default behavior otherwise. Since there can be a great deal of those functions with a particular design pattern (like a CRTP that reads a protocol with many different cases and processes a case a particular way only if the derived class specify the corresponding function, otherwise just transfer it without processing).

So those functions can be present in SubBattleData and not BattleData, but both classes would work fine if instantiated, yet it’s impossible to instantiate BattleData.

Answer 1

Here is how I solved it:

template <class Derived> class BattleDataInh: public BaseClass<Derived> {
    void foo1(){};
    void foo2(){};
    void foo3(){};
};

template class SubBattleData: public BattleDataInh<SubBattleData> {
    void foo1(){};
};

class BattleData : public BattleDataInh<BattleData> {
};

And that way, I can add any other template parameters too. The solution was in front of my eyes the whole time but I didn’t see it…