Confusing Inheritance and templates

Question

Here is the code I was looking, Source code :

template <typename T>
struct function_traits
    : public function_traits<decltype(&T::operator())>
{};

if we instantiate it with some functor X i.e function_traits<X>; , That will build the base class which is function_traits<decltype(&X::operator())> due to inheritance , but to build function_traits<decltype(&X::operator())> it's base also has to be built, which could be function_traits<decltype(Z)>

I understand function_traits<X> != function_traits<Z>. Isn't that recursive inheritance? 0_o. How all things is working together?

I don't understand what your question is. This isn't valid code without some other specialization of function_traits for member function pointers. — ildjarn
– ildjarn, Commented Dec 21, 2011 at 21:15
@FreakEnum : Yes, that code contains the exact specializations I was referring to. ;-] — ildjarn
– ildjarn, Commented Dec 21, 2011 at 21:28
What are you trying to do? Can you show us a minimal working example? — curiousguy
– curiousguy, Commented Dec 26, 2011 at 0:54

Peter Alexander · Accepted Answer · 2011-12-21 21:30:15Z

2

This is illegal code. You cannot derive from an incomplete type, and at the point you try to derive from function_traits, the type is incomplete.

struct A { typedef A type; };
struct B { typedef A type; };

template <typename T>
struct X : X<typename T::type> {};

X<B> test; // error: invalid use of incomplete type ‘struct X<A>’

The only way you could get round this is if the function_traits you are trying to derive from is a complete type. You can do this using specialisation:

struct A { typedef A type; };
struct B { typedef A type; };

template <typename T>
struct X : X<typename T::type> {};

template <>
struct X<A> {}; // X<A> is now a complete type.

X<B> test; // OK! Derives from X<A>, which is complete.

Here, X<A> is complete when you try to derive from it, so you're fine.

edited Dec 21, 2011 at 21:30

answered Dec 21, 2011 at 21:22

Peter Alexander

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6 Comments

Mr.Anubis Over a year ago

here is the whole If you might wanna look github.com/kennytm/utils/blob/master/traits.hpp

Peter Alexander Over a year ago

@FreakEnum: As you can see, he specialises the template for various types of functions below the first definition, so he always has a complete class. I'm sure KennyTM will answer any other questions you have :-) stackoverflow.com/users/224671/kennytm

Mr.Anubis Over a year ago

Now that made sense to me. Thank you:)

curiousguy Over a year ago

"This is illegal code." The code shown is incomplete. How can you say it is not legal? "the type is incomplete" But what's a complete type? We do not have enough information.

Peter Alexander Over a year ago

@curiousguy: True, but when I posted there was no link to the source, so I assumed it was the whole code.

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