CODEWITHSUNDEEP
templatesc++11types
user695652
user695652

Reputation: 4275

Choosing a parameter type based on template parameter

I am writing a C++11 template function void foo(T, U) taking two parameters:

The first parameter which can either be of type A1 or of type B1. If the first parameter is of type A1, the second parameter is of type A2; If the first parameter is of type B1, the second parameter is of type B2;

Since the second parameter depends on the first one, is there a way of writing this function needing only one template parameter?

Something along the line of 

template class <T>
void foo(T t, std::conditional<A1* tmp = dynamic_cast<T*>(&t), A2, B2>::type);

might work, but its ugly and needs RTTI.

Is there a good way of achieving this?

Upvotes: 4

Views: 84

Answers (1)

ROX
ROX

Reputation: 1266

Yes, it can be done without RTTI. how best to do it depends on how A1 and A2 are related.

If you control the classes you can add a using or typedef into A1

class A1
{
     using RelatedType = A2;
}

and similar for B1 and B2

then 

template class <T>
void foo(T t, T::RelatedType t2);

If you cant edit A1 you can create a type trait

template<typename T>
struct TypeRelation  // would have a better name if we knew why they were related
{
      // can put a default related type here if you want
}

and then use template specialization (example for As, but need similar for Bs)

template<>
struct TypeRelation<A1>
{
     using RelatedType = A2;
}

and then the declaration becomes

template class <T>
void foo(T t, typename TypeRelation<T>::RelatedType t2);

C++11s decltype may also be suitable depending on how the classes are related.

Upvotes: 1

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