C++ template class inherit itself

Question

I have some trouble in reading some source codes from Eigen library. Here is the base class TensorBase:

template<typename Derived>
class TensorBase<Derived, ReadOnlyAccessors> {
  ...
};

In the following code, How can the base class TensorBase inherit itself? What's the point of using this inheritance? Is this common?

template<typename Derived, int AccessLevel = 
         internal::accessors_level<Derived>::value>
class TensorBase : public TensorBase<Derived, ReadOnlyAccessors> {
  ...
};

Is there any chance it is similar with this? https://stackoverflow.com/questions/46236565/how-can-a-class-inherit-itself/46236632 or https://stackoverflow.com/questions/8336220/how-can-a-class-inherit-from-a-template-based-on-itself — aliyasineser, Oct 23 '18 at 07:05
A class cannot inherit itself indeed, but TensorBase is not a class. It's a class template. A specialisation of a class template can inherit another specialisation of the same template. — n. m. could be an AI, Oct 23 '18 at 07:06
@aliyasineser: The first link just answer my question. Thanks! — Yulong Ao, Oct 23 '18 at 07:27

463035818_is_not_an_ai · Accepted Answer · 2018-10-23T07:46:41.243

The class is not inheriting from itself. It is inheriting from a different instantiation of the same template. Remember, different instantiations of the same template are different types.

For illustration, consider this simple albeit contrived example:

#include <iostream>
using namespace std;

template <int N>
struct foo : foo<N-1> {};

template <> 
struct foo<0> {
    static const int value = 23;
};


int main() {
    std::cout << foo<23>::value;
    return 0;
}

foo<23> inherits from foo<22> which in turn inherits from foo<21> and so on until eventually foo<0> does not inherit from anything.

In your case it seems like read-only functionality is implemented in a base class that is inherited from the instantiations for different access levels.

C++ template class inherit itself

1 Answers1