5

I have multiple factories, each is responsible for creating instances of an inheritance tree. For example (syntax or semantics maybe incorrect),

struct InterfaceA {};  struct ImplA1 : InterfaceA {}; ... 
struct FactoryA { vector<InterfaceA*> create(); }    
... // (repeat for B, C, etc)

foo is template class

template<ATy, BTy, ..>
struct foo {
     // default behavior - no functionality
};

foo is specialized for some combination of subclasses of InterfaceA, InterfaceB, and ...

template<>
struct foo<ImplA1, ImplB2, ..> {
     void feature1();
     void feature2();
};

Is it possible to combine the inheritance hierarchies with the template class? i.e.

void bar(vector<InterfaceA*> vecA, vector<InterfaceB*> vecB, ..) {
    foo< /* what to put in here */ >(vecA.front(), vecB.front(), ...) f;
    f.feature1();  // if specialization not there, complain
}

I can use Curiously Recurring Template Pattern (CRTP) for one tree. Not sure how to get it work with multi trees.

  • 2
    I'm not sure what problem you're trying to solve. The static types of vecA.front() and vecB.front() are clearly known. And if you need to instantiate a template based on their dynamic type – well, you can't do that… – 5gon12eder May 18 '16 at 20:49
  • vecA.front() is InterfaceA*? – Candy Chiu May 18 '16 at 21:10
  • @CandyChiu: Yes, vecA.front() returns an item of type InterfaceA*. It can point to an ImplA1, ImplA2 or any other derived class of InterfaceA. – Bart van Ingen Schenau May 19 '16 at 9:08
  • Yes, so in template resolution, it won't select the specialized. – Candy Chiu May 19 '16 at 13:08
1

Seems what you want is:

void bar(vector<InterfaceA*> vecA, vector<InterfaceB*> vecB) {
    // If InterfaceA == ImplA1 && InterfaceB == ImplB2 then:
    foo<ImplA1, ImplB2> f(*dynamic_cast<const ImplA1*>(vecA.front()), *dynamic_cast<const ImplB2*>(vecB.front()));
    // ...
    f.feature1();  // if specialization not there, complain
}

which is not possibly, because the template instantiation is a compile-time thing and you want to make a decision on a dynamic run time type. (Btw. Even above only works if you give InterfaceA|B a virtual d'tor.)

So from your comment // if spec not there, complain I can state this:

  • You cannot complain at compile time, because the decision to make is based on run time information
  • You could write a runtime dispatcher that selects the correct template instantiation (or fails with e.g. an exception) at run time: How to do that is another story, possibly best asked over at SO.

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