How to generate a member function with a deduced signature

Question

is it possible to produce method definition (with an exact number of parameters and return value of known type), having:

• method argument types "frozen" in a variadic arguments pack
• deduced method return type
• method name (passed to a macro?)

Details:

I have a simple reflection structure (partial specialisation stuff omited for readability), which deduces member functions return type and argument types:

template<typename RetType, typename ...ArgTypes>
struct reflect_method<RetType(HostClassType::*)(ArgTypes...)> {
  using method_type = RetType;
  using method_args = type_placeholder<ArgTypes...>;
  using call_type = RetType(HostClassType::*)(ArgTypes...);
};

where method_type is a method return type, method_args are a method argument types "frozen" in a helper template struct type_placeholder.

What I'm trying to do is to create a method in a generated class which will reflect arguments and return type of another method of some other class. The created method will provide a decoration for the reflected method.

Pseudocode implementation:

#define RPCCLASS(class_name)    class RPC##class_name : public class_name   \
                                {                                           \
                                  using SelfType = RPC##class_name;         \
                                  using ParentType = class_name;

#define RPCCLASS_END()          };




#define RPCBIND(method_name)   \
    using method_name_##tag = reflect_method<decltype(ParentType::method_name)>; \
    method_name_##tag::method_type
    method_name(method_name_##tag::method_args::at<0>::type arg0, \
                method_name_##tag::method_args::at<1>::type arg1, \
                /* ... */                                         \
                /*don't know how to put correct number of arguments here)*/)    \
    {                                                                           \
      /* do some stuff */                                                       \
      /* ... */                                                                 \
      /* invoke the reflected method */                                         \
      return Invoke<method_name_##tag>::apply(this, method_name,                \
                                              arg0,                             \
                                              arg1                              \
             /*again don't know how to put correct number of arguments here)*/) \
     }


 // USAGE:
 class MyOwnClass {
 public:
   virtual long long doFun(int a, char b, const std::string& c);
 };


 RPCCLASS(MyOwnClass)
   RPCBIND(doFun)
 RPCCLASS_END()

Answer 1

I found a solution. Instead of trying to generate a member function to reflect a decorated method I figured out that I can generate a member functor instead. The functor is realised as a template structure with a operator()(...). This allows me to have specialisations with correct number of arguments while retaining member function call semantics.

Sample code :

template <int Count>
struct apply_placeholders
{
};

template <>
struct apply_placeholders<1>
{
  template<typename CallType, typename Self, template<typename>class CallPtrType>
  static CallType apply(Self* self, CallPtrType<Self> callPtr)
  {
    return std::bind(std::mem_fn(callPtr), self, std::placeholders::_1);
  }
};
template <>
struct apply_placeholders<2>
{
  template<typename CallType, typename Self, template<typename>class CallPtrType>
  static CallType apply(Self* self, CallPtrType<Self> callPtr)
  {
    return std::bind(std::mem_fn(callPtr), self, std::placeholders::_1, std::placeholders::_2);
  }
};
template <>
struct apply_placeholders<3>
{
  template<typename CallType, typename Self, template<typename>class CallPtrType>
  static CallType apply(Self* self, CallPtrType<Self> callPtr)
  {
    return std::bind(std::mem_fn(callPtr), self, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3);
  }
};
template <>
struct apply_placeholders<4>
{
  template<typename CallType, typename Self, template<typename>class CallPtrType>
  static CallType apply(Self* self, CallPtrType<Self> callPtr)
  {
    return std::bind(std::mem_fn(callPtr), self, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3, std::placeholders::_4);
  }
};

template<typename RetType, template<typename...>class FrozenArgTypes, typename... ArgTypes>
struct mimic_functor_impl
{
};

template<typename RetType, typename... ArgTypes>
struct mimic_functor_impl<RetType, type_placeholder, type_placeholder<ArgTypes...>>
{
public:
  using CallType = std::function<RetType(ArgTypes...)>;
  template<typename Self>
  using CallPtrType = RetType(Self::*)(ArgTypes...);
private:
  CallType    mCall;

public:
  mimic_functor_impl(CallType call) : mCall{call}
  {
  }

  RetType operator () (ArgTypes... args)
  {
    return mCall(args...);
  }

  template<typename Self>
  static CallType make_function(Self* self, CallPtrType<Self> callPtr)
  {
    // manually specialise the template method because the compiler get's lost on matching "Self::*" in CallPtrType
    return apply_placeholders<sizeof...(ArgTypes)>::template apply<CallType, Self, CallPtrType>(self, callPtr);
  }
};

template<typename... ArgTypes>
struct mimic_functor_impl<void, type_placeholder, type_placeholder<ArgTypes...>>
{
public:
  using CallType = std::function<void(ArgTypes...)>;
  template<typename Self>
  using CallPtrType = void(Self::*)(ArgTypes...);
private:
  CallType        mCall;

public:
  mimic_functor_impl(CallType call) : mCall{call}
  {
  }

  void operator () (ArgTypes... args)
  {
    mCall(args...);
  }
  template<typename Self>
  static CallType make_function(Self* self, CallPtrType<Self> callPtr)
  {
    // manually specialise the template method because the compiler get's lost on matching "Self::*" in CallPtrType
    return apply_placeholders<sizeof...(ArgTypes)>::template apply<CallType, Self, CallPtrType>(self, callPtr);
  }
};

template<typename Reflect>
struct mimic_functor : mimic_functor_impl<typename Reflect::method_type, type_placeholder, typename Reflect::method_args>
{
private:
  using BaseType = mimic_functor_impl<typename Reflect::method_type, type_placeholder, typename Reflect::method_args>;
public:
  mimic_functor(typename BaseType::CallType call) : BaseType(call)
  {
  }
};



#define __TAG(x)     x ## _tag

#define RPCBIND(method_name) \  
        public: \
        using __TAG(method_name) = reflect_method<decltype(&ParentType::method_name)>; \
        mimic_functor<__TAG(method_name)> method_name{mimic_functor<__TAG(method_name)>::make_function(dynamic_cast<ParentType*>( const_cast<SelfType*>( this ) ), &ParentType::method_name)};

The rest of the code is like in the question listing.

The apply_placeholders template expands a required number of placeholders to match the count of parameters in a variadic parameter pack.

The mimic_functor_impl and mimic_functor templates create the functor whose operator ()(...) will match the reflected methods signature. The functor, when invoked, also calls the reflected method.

The make_function member template function creates a bound std::function containing the reflected method with "this" pointer.