Lazy evaluation

Question

How can I lazy evaluate second arg in std::conditional?

#include \"stdafx.h\"
#include 

struct Null{};
struct _1{enum {one = true,two = fal         


        

Answer 1

The usual technique for this is to have the std::conditional choose between two metafunctions:

template <typename T>
struct false_case {
  typedef typename std::conditional<T::one,_1,_2>::type type;
};

struct always_null {typedef Null type;};

template<class T>
struct X :
  std::conditional<is_nulltype<T>::value,
                   always_null,
                   false_case<T>
                  >::type::type { ... };

Note the two ::types after std::conditional now.