boost mpl if demonstration

Consider the following code adapted from Polukhin:

	#include <boost/mpl/if.hpp>
	#include <boost/type_traits/has_plus_assign.hpp>
	#include <boost/type_traits/has_plus.hpp>
	#include <boost/type_traits/has_post_increment.hpp>
	#include <boost/type_traits/has_pre_increment.hpp>
	#include <cstdio>
	namespace detail {
	struct pre_inc_functor {
	template <class T>
	void operator()(T& value) const {
	printf("%s\n", __FUNCTION__);
	++value;
	}
	};
	struct post_inc_functor {
	template <class T>
	void operator()(T& value) const {
	printf("%s\n", __FUNCTION__);
	value++;
	}
	};
	struct plus_assignable_functor {
	template <class T>
	void operator()(T& value) const {
	printf("%s\n", __FUNCTION__);
	value += T(1);
	}
	};
	struct plus_functor {
	template <class T>
	void operator()(T& value) const {
	printf("%s\n", __FUNCTION__);
	value = value + T(1);
	}
	};
	}
	template <class T>
	void inc(T& value) {
	typedef detail::plus_functor step_0_t;
	typedef typename boost::mpl::if_<
	boost::has_plus_assign<T>,
	detail::plus_assignable_functor,
	step_0_t
	>::type step_1_t;
	typedef typename boost::mpl::if_<
	boost::has_post_increment<T>,
	detail::post_inc_functor,
	step_1_t
	>::type step_2_t;
	typedef typename boost::mpl::if_<
	boost::has_pre_increment<T>,
	detail::pre_inc_functor,
	step_2_t
	>::type step_3_t;
	step_3_t() // default constructing functor
	(value); // calling operator() of a functor
	}
	class cls
	{
	public:
	cls(int val): m_int(val) {}
	cls operator+=(cls const& rhs)
	{
	printf("%s\n", __FUNCTION__);
	m_int += rhs.m_int;
	return *this;
	}
	int get()
	{
	return m_int;
	}
	private:
	int m_int;
	};
	class post
	{
	public:
	post(int val): m_int(val) {}
	post operator++(int)
	{
	printf("%s\n", __FUNCTION__);
	post tmp(m_int);
	++m_int;
	return tmp;
	}
	int get()
	{
	return m_int;
	}
	private:
	int m_int;
	};
	int main()
	{
	int x = 5;
	inc(x);
	printf("x = %d\n", x);
	cls y(89);
	inc(y);
	printf("y = %d\n", y.get());
	post z(140);
	inc(z);
	printf("z = %d\n", z.get());
	post alpha = z++;
	printf("alpha = %d\n", alpha.get());
	printf("z = %d\n", z.get());
	return 0;
	}

view raw b.cpp hosted with ❤ by GitHub

When run:

detail::pre_inc_functor::operator ()
x = 6
detail::plus_assignable_functor::operator ()
cls::operator +=
y = 90
detail::post_inc_functor::operator ()
post::operator ++
z = 141
post::operator ++
alpha = 141
z = 142