C++: Partially specializing template's type parameter as another template class's member-type

Question

I have a template struct SFoo that contains a member struct SZug:

template <typename tTYPE>
struct SFoo
  {
    struct SZug {};
  };

I have another struct SBar that takes a type parameter:

template <typename tTYPE>
struct SBar
  { /* stuff */ };

I would like to specialize SBar using SZug for the type parameter, like so:

template <typename tTYPE>
struct SBar<typename SFoo<tTYPE>::SZug>
  { /* different stuff */ };

This doesn't compile - LLVM outputs:

non-deducible template parameter 'tTYPE'

While a compiler could easily deduce this if it wished, I'm guessing it's just that the C++ spec would need to specifically cover this case.

Is there any way to achieve this?
(note: I'm currently working around it by moving SZug outside of SFoo and using a using declaration, but it's ugly.)

Answer 1

I am not sure I fully understood what you want to do, but you could try the following (it only requires adding a specific attributes to SZug:

template <typename tTYPE>
struct SFoo {
    struct SZug {
        // Add this to be able to obtain SFoo<T> from SFoo<T>::SZug
        using type = tTYPE;
    };
};

Then a small template to check if a type is a SFoo<T>::SZug:

template <typename tTYPE, typename Enabler = void>
struct is_SZug: public std::false_type { };

template <typename tTYPE>
struct is_SZug<tTYPE, typename std::enable_if<
  std::is_same<tTYPE, typename SFoo<typename tTYPE::type>::SZug>{}
>::type>: public std::true_type { };

And a slight modification to the SBar template to enable the "specialization" if the type is a SZug:

template <typename tTYPE, typename Enabler = void>
struct SBar
  { static void g(); };

template <typename tTYPE>
struct SBar<tTYPE, typename std::enable_if<is_SZug<tTYPE>{}>::type>
  { static void f(); };

A little check:

void f () {
  SBar<int>::g();
  SBar<SFoo<int>::SZug>::f();
}

Note: You could also directly set SFoo<T> as the type attribute in SFoo<T>::SZug, you would simply need to change the second argument of std::is_same a little.

Answer 2

You can get the effect for which you're looking through the following (which prints out 0 1, BTW):

#include <type_traits>
#include <iostream>

namespace detail
{   
    struct SZugBase{};
}   

template <typename tTYPE>
struct SFoo                                                                                                                                
{   
    struct SZug : public detail::SZugBase {}; 
};  

template<typename tType, bool IsFoo>
struct SBarBase
{   
    int value = 0;
};  

template<typename tType>
struct SBarBase<tType, true>
{   
    int value = 1;
};  

template <typename tTYPE>
struct SBar : public SBarBase<tTYPE, std::is_convertible<tTYPE, detail::SZugBase>::value>
{ /* stuff */ };

int main()
{   
    SBar<int> b0; 
    SBar<SFoo<int>::SZug> b1; 

    std::cout << b0.value << " " << b1.value << std::endl;
}   

Explanation

First, we give SZug a regular-class base:

namespace detail
{   
    struct SZugBase{};
}   

template <typename tTYPE>
struct SFoo                                               
{   
    struct SZug : public detail::SZugBase {}; 
};  

Note the following:

1. SZugBase is not parameterized by anything, so it is easy to refer to it independently of the parameter of SFoo

2. SZugBase is in a detail namespace, so, by common C++ conventions, you're telling clients of your code to ignore it.

Now we give SBar two base classes, specialized on whether something is convertible to the non-template base of SZug:

template<typename tType, bool IsFoo>
struct SBarBase
{   
    int value = 0;
};  

template<typename tType>
struct SBarBase<tType, true>
{   
    int value = 1;
};  

Finally, we just need to make SBar a subclass of these bases (depending on the specialization):

template <typename tTYPE>
struct SBar : public SBarBase<tTYPE, std::is_convertible<tTYPE, detail::SZugBase>::value>
{ /* stuff */ };

Note that you don't specialize SBar here, you rather specialize the base classes. This effectively gives the same effect, though.