Getting class and member type from pointer to member variable

Question

For example:

template <typename T>
void foo(T ptr)
{
  typedef GET_CLASS_TYPE(T) ClassT;
  typedef GET_VALUE_TYPE(T) ValueT;
  // ...
}

struct Bar
{
  int var;
};

foo(&Bar::var);  

Inside the last function call to foo(...), ClassT should be Bar and ValueT should be int.

How can I do this with plain C++ (can't use C++11 features), or boost?

Answer 1

Not aware of any out-of-the-box Boost type trait to do this, but the same can be written by yourself using template specialization:

template<typename T>
struct member_pointer_class;

template<typename Class, typename Value>
struct member_pointer_class<Value Class::*>
{
    typedef Class type;
};

template<typename T>
struct member_pointer_value;

template<typename Class, typename Value>
struct member_pointer_value<Value Class::*>
{
    typedef Value type;
};

// TEST
#include <boost/type_traits.hpp>
#include <boost/static_assert.hpp>

struct Bar
{
    int var;
};

template <typename T>
void foo(T ptr)
{
    // test the code
    typedef typename member_pointer_class<T>::type ClassT;
    typedef typename member_pointer_value<T>::type ValueT;
    BOOST_STATIC_ASSERT_MSG((boost::is_same<ClassT, Bar>::value), "member_pointer_class is the same as Bar");
    BOOST_STATIC_ASSERT_MSG((boost::is_same<ValueT, int>::value), "member_pointer_value is the same as int");
}

int main()
{
    foo(&Bar::var);
}

Explanation:

Using template deduction we extract the interesting types of the member pointer - the typedefs member_pointer_class<T>::type and member_pointer_value<T>::type are defined to the appropriate types. typename is required for disambiguation in templates. The code can be also used for pointer to member functions.

If the type is not a pointer to member, the member_pointer_class<T>::type gives a compiler error.