Question

I wonder if it is possible using boost::mpl/preprocessor or some noce C++11 features to create function proxy from class type and function name.

Say we had:

  inline void set_email(const ::std::string& value);
  inline void set_email(const char* value);

inside class Email. We know there is set_email function n it, we want to create a prox class with API like

PROXY(Email, set_email, MyEmail)

Email * email = new Email();
MyEmail * myEmail = new MyEmail(email);

and have abilety to call any of set_email overloads.Is it possible and how to create such class that would proxy any number of overload functions not knowing there types (only names)?

Answer 1

How about this:

proxy_macro.hpp

#include <type_traits>
#include <utility>

#define PROXY(proxified, member_function, proxy_name)                                                                           \
  class proxy_name                                                                                                              \
  {                                                                                                                             \
  private:                                                                                                                      \
    proxified & ref_;                                                                                                           \
                                                                                                                                \
  public:                                                                                                                       \
    proxy_name(proxified &ref)                                                                                                  \
      : ref_(ref)                                                                                                               \
      {                                                                                                                         \
      }                                                                                                                         \
                                                                                                                                \
    /* general version */                                                                                                       \
    template<typename ... Args>                                                                                                 \
    auto member_function(Args&& ... args)                                                                                       \
    -> typename std::enable_if<!std::is_void<decltype(ref_.member_function(std::forward<Args>(args)...))>::value,               \
                               decltype(ref_.member_function(std::forward<Args>(args)...))>::type                               \
      {                                                                                                                         \
        return (ref_.member_function(std::forward<Args>(args)...));                                                             \
      }                                                                                                                         \
                                                                                                                                \
    /* void return type version */                                                                                              \
    template<typename ... Args>                                                                                                 \
    auto member_function(Args&& ... args)                                                                                       \
    -> typename std::enable_if<std::is_void<decltype(ref_.member_function(std::forward<Args>(args)...))>::value,                \
                               void>::type                                                                                      \
      {                                                                                                                         \
        ref_.member_function(std::forward<Args>(args)...);                                                                      \
      }                                                                                                                         \
                                                                                                                                \
  };

This compiles and work fine for me on g++ 4.7:

#include "proxy_macro.hpp"

#include <iostream>
#include <string>

class   Email
{
public:  
  void set_email(const ::std::string& value)
  {
    std::cout << value << std::endl;
  }

  void set_email(const char* value)
  {
    std::cout << value << std::endl;
  }

  int   set_email()
  {
    return (42);
  }

};

PROXY(Email, set_email, MyEmail)

int main(void)
{
  Email   mail;
  MyEmail my_mail(mail);

  std::string str = "test string";
  const char * ptr = "test char pointer";

  my_mail.set_email(str);
  my_mail.set_email(ptr);

  std::cout << "test return: " << my_mail.set_email() << std::endl;

  return (0);
}

---

Edit (smaller version thanks to comments)

proxy_macro.hpp

#include <type_traits>
#include <utility>

#define PROXY(proxified, member_function, proxy_name)                \
  class proxy_name                                                   \
  {                                                                  \
  private:                                                           \
    proxified & ref_;                                                \
                                                                     \
  public:                                                            \
    proxy_name(proxified &ref)                                       \
      : ref_(ref)                                                    \
      {                                                              \
      }                                                              \
                                                                     \
    template<typename ... Args>                                      \
    auto member_function(Args&& ... args)                            \
    -> decltype(ref_.member_function(std::forward<Args>(args)...))   \
      {                                                              \
        return (ref_.member_function(std::forward<Args>(args)...));  \
      }                                                              \
  };