templates - How to check whether a class has specified nested class definition or typedef in C++ 11?

Question

In my project, I want to implement a template proxy class of some existing bigger classes. The existing classes are library classes, so they can not be modified. In most cases, the clients do not know the objects are instances of proxy class or bigger class. In some cases, however, the clients MUST know the detailed class information. Since the proxy class is itself a template class, I do not think simple function overloading by class name could solve this problem. The possible solution I thought is to add an internal nested class or typedef inside the proxy class, and the client check whether this class/typedef exists to get the class information. My question is: how to check whether a class has specified nested class definition or typedef in C++ 11 ?

The following codes show an example:

#include <iostream>
#include <functional>
#include <string>
#include <vector>
#include <type_traits>

typedef std::string CBig1;  //  use string for demonstration
typedef std::string CBig2;  //  use string for demonstration

//class CBig1;   // the bigger class 1, codes of which can not be changed
//class CBig2;   // the bigger class 2, codes of which can not be changed

template <typename _Big, typename _Other>
class CProxy
{
public:
    struct proxy_tag { };
};

//  how to implement this ?
//  the proxy traits class, if defined _T::proxy_tag, the ``type'' will be std::true_type, otherwise the ``type'' will be std::false_type
template <typename _T>
struct is_proxy
{
    //typedef std::true_type type;
    //typedef std::false_type type;
};

template <typename _T>
void ClientHelp(const _T& t, std::false_type)
{
    //  process real class
    std::cerr << "real class" << std::endl;
}

template <typename _T>
void ClientHelp(const _T& t, std::true_type)
{
    //  process proxy class
    std::cerr << "proxy class" << std::endl;
}

template <typename _T>
void Client(const _T& t)
{
    ClientHelp(t, typename is_proxy<_T>::type());
}

int main(int argc, char* argv[])
{
    CBig1 b;
    CProxy<CBig1, int> p;
    Client(b);
    Client(p);
    return 0;
}

How to implement the traits class is_proxy?

Answer 1

You can use the lightweight type categorization idiom

template<class T, class R = void>  
struct enable_if_type { typedef R type; };

template<class T, class Enable = void>
struct test : std::false_type {};

template<class T>
struct test<T, typename enable_if_type<typename T::is_proxy_tag>::type> : std::true_type
{};

template <typename _Big, typename _Other>
class CProxy
{
  public:
  typedef void is_proxy_tag;
};

So to make a class a Proxy, just add this

typedef void is_proxy_tag;

and the SFINAE in enable_if_type will select the proper true_type/false_type specialisation

Note that using boost::mpl::true_ instead of std::true_type etc makes this solution work for C++03.