Hello, I am going through the book C++ Template Metaprogramming and wanted a sanity check on Exercise 2-0 from the book. My first question is: What would be a motivating example for why one would want a add_const_ref<T> metafunction that returned T if it was a reference and T const& otherwise? My second question is: How did I do? My guess is that the second crack is more correct. Here is crack one: My first crack at the exercise is below (my 2nd crack is below that) #include <iostream> #include <boost/type_traits/is_reference.hpp> #include <boost/type_traits/is_same.hpp> #include <boost/static_assert.hpp> using namespace std; template<typename T, bool> struct add_const_ref { typedef T const& value_type; }; template<typename T> struct add_const_ref<T, true> { typedef T value_type; }; template<typename T> void func(typename add_const_ref<T, boost::is_reference<T>::value
::value_type t)
{ // .. do something } class BigClass { }; int main() { int i = 0; BOOST_STATIC_ASSERT((boost::is_same<add_const_ref<int&, boost::is_reference<int&>::value >::value_type, int&>::value)); BOOST_STATIC_ASSERT((boost::is_same<add_const_ref<int, boost::is_reference<int>::value >::value_type, int const&>::value)); func<int&>(i); BigClass myBigClass; func<BigClass>(myBigClass); return 0; } After taking a look at add_reference.hpp in boost I modified my answer to the one below: #include <iostream> #include <boost/type_traits/is_reference.hpp> #include <boost/type_traits/is_same.hpp> #include <boost/static_assert.hpp> using namespace std; template<bool> struct const_ref_adder { template<typename T> struct result_ { typedef T const& value_type; }; }; template<> struct const_ref_adder<true> { template<typename T> struct result_ { typedef T value_type; }; }; template<typename T> struct add_const_ref { typedef typename const_ref_adder<boost::is_reference<T>::value
::template result_<T>::value_type value_type;
}; template<typename T> void func(typename add_const_ref<T>::value_type t) { // .. do something } class BigClass { }; int main() { int i = 0; BOOST_STATIC_ASSERT((boost::is_same<add_const_ref<int&>::value_type, int&>::value)); BOOST_STATIC_ASSERT((boost::is_same<add_const_ref<int>::value_type, int const&>::value)); func<int&>(i); BigClass myBigClass; func<BigClass>(myBigClass); return 0; } Thanks in advance for your input, Bruce