Enable template only for specific templated class

Question

This question is inspired from my previous question No template parameter deduction of parameter pack.

Consider following code example:

#include <memory>
#include <string>

template<typename... FArgs>
class Callback
{
    public:
    class Handle{};
};

class BaseCallbackHandle
{
};

using TypeErasedCallbackHandle = std::unique_ptr<BaseCallbackHandle>;

template<typename H>
TypeErasedCallbackHandle makeTypeErasedCallbackHandle( H handle)
{
   return {};
}

int main()
{
    Callback<int>::Handle h;
    std::string s;
    makeTypeErasedCallbackHandle(h); //should compile fine
    makeTypeErasedCallbackHandle(s); //should raise a compile error
}

See also http://coliru.stacked-crooked.com/a/5f2a2e816eef6afd

The function template makeTypeErasedCallbackHandle now takes any class as input parameter. Is there any way to ensure (e.g. with static-assert or enable_if), that only Callback<FArgs...>::Handle (with any FArgs) is allowed as H? The example with Callback<int>::Handle shall compile, while std::string shall fail.

Answer 1

Define a type within your Handle class, and refer to that type inside makeTypeErasedCallbackHandle():

#include <memory>
#include <string>

template <typename... FArgs>
struct Callback {
    struct Handle {
        using callback_type = Callback<FArgs...>;    
    };
};

struct BaseCallbackHandle {
};

using TypeErasedCallbackHandle = std::unique_ptr<BaseCallbackHandle>;

template <typename H>
TypeErasedCallbackHandle makeTypeErasedCallbackHandle(H handle) {
    using callback_type = typename H::callback_type;

    return {};
}

int main() {
    Callback<int>::Handle h;
    std::string s;
    makeTypeErasedCallbackHandle(h); //should compile fine
    makeTypeErasedCallbackHandle(s); //should raise a compile error
}

Live example

This will fail during instantiation for any H that doesn't define the nested type.

---

With a little more effort, you can static_assert to produce a meaningful message to the client, while at the same time increasing the flexibility of the solution via type traits. This has the advantage that callback_impl::is_callback can be specialised for arbitrary handle types:

#include <memory>
#include <string>

namespace callback_impl {

struct callback_identification_type {};

template <typename T, typename = void>
struct is_callback : std::false_type {};

template <typename T>
struct is_callback<T,
    std::enable_if_t<std::is_same<typename T::callback_id_type,
                     callback_identification_type>::value>>
 : std::true_type {};

}

template <typename... FArgs>
struct Callback {
    struct Handle {
        using callback_id_type = callback_impl::callback_identification_type;    
    };
};

struct BaseCallbackHandle {
};

using TypeErasedCallbackHandle = std::unique_ptr<BaseCallbackHandle>;

template <typename H>
TypeErasedCallbackHandle makeTypeErasedCallbackHandle(H handle) {
    static_assert(callback_impl::is_callback<H>::value,
                  "The handle type is not a member of a recognised Callback<T...>");
    return {};
}

int main() {
    Callback<int>::Handle h;
    std::string s;
    makeTypeErasedCallbackHandle(h); //should compile fine
    makeTypeErasedCallbackHandle(s); //should raise a compile error

    return 0;
}

Live example

Output:

g++ -std=c++14 -O2 -Wall -Wno-unused-local-typedefs -pedantic -pthread main.cpp && ./a.out
main.cpp: In instantiation of 'TypeErasedCallbackHandle makeTypeErasedCallbackHandle(H) [with H = std::__cxx11::basic_string<char>; TypeErasedCallbackHandle = std::unique_ptr<BaseCallbackHandle>]':
main.cpp:41:35:   required from here
main.cpp:32:5: error: static assertion failed: The handle type is not a member of a recognised Callback<T...>
     static_assert(callback_impl::is_callback<H>::value,
     ^~~~~~~~~~~~~