One of the practical issues with C++ meta-programming is its speed. C++ programs that use heavy meta-programming can be notoriously slow to compile on contemporary compilers. Things are changing, however. Check the following comparison of gcc 4.5 against gcc 4.4.3.

The first graph is obtained from a program that creates a binary tree of template instantiations. The x-axis shows the number of instantiations when value of N goes from 8 to 17. I could not build up patience for gcc 4.4.3 beyond 16363 instantiations (N=13). On the other hand, gcc 4.5 does pretty good and its increase in compilation time is indeed linear as mentioned here. Here is the program that creates a binary tree of template instantiations.

So long story short, seems like better days are ahead for C++ meta-programming!

The first graph is obtained from a program that creates a binary tree of template instantiations. The x-axis shows the number of instantiations when value of N goes from 8 to 17. I could not build up patience for gcc 4.4.3 beyond 16363 instantiations (N=13). On the other hand, gcc 4.5 does pretty good and its increase in compilation time is indeed linear as mentioned here. Here is the program that creates a binary tree of template instantiations.

template <int Depth, int A, typename B> struct Binary { enum { value = 1 + Binary<depth-1, 0, Binary>::value + Binary<depth-1, 1, Binary>::value }; }; template<int a, typename B> struct Binary<0, A, B> { enum { value = 1 }; }; int main(void) { static const int N = 10; const int instantiations = Binary<N,0,int>::value; }The second graph is obtained from a program that finds an intersection of two MPL vectors. Again gcc 4.5 shows linear increase in compilation time as opposed to gcc 4.4.3. Here is the intersection program.

template <class V1, class V2> struct Intersection { typedef typename boost::mpl::copy_if<V1, boost::mpl::contains<V2, boost::mpl::placeholders::_1> >::type type; };While all that is already exciting, it fades in comparison to the performance of variadic templates in C++0x. The green line in the second graph shows negligible effect on performance with the increasing number of template parameters. Here is my intersection metaprogram using variadic templates.

struct null_type {}; template <typename... Arg> struct vector {}; template <typename V> struct front; template <typename V> struct pop_front; template <typename Head, typename... Tail> struct front <vector <Head, Tail...> > { typedef Head type; }; template <> struct front <vector <> > { typedef null_type type; }; template <typename Head, typename... Tail> struct pop_front <vector <Head, Tail...> > { typedef vector<Tail...> type; }; template <> struct pop_front <vector <> > { typedef vector<> type; }; template <typename Vector, typename T> struct push_back; template <typename T, typename... Args> struct push_back < vector<Args...>, T> { typedef vector<Args..., T> type; }; template <typename Vector> struct size; template <typename... Args> struct size <vector <Args...> > { typedef size type; enum { value = sizeof...(Args) }; }; template <typename Vector, typename What> struct contains; template <typename What, typename Head, typename... Tail> struct contains < vector<Head, Tail...>, What> : std::conditional < std::is_same<Head, What>::value, std::true_type, contains < vector<Tail...>, What> >::type { typedef contains type; }; template <typename What> struct contains <vector<>, What> { typedef contains type; enum { value = 0 }; }; template <class V1, class V2> struct Intersection; template <class V1, class V2, unsigned int N> struct Intersection_impl { typedef typename front<V2>::type Head; typedef typename pop_front<V2>::type Tail; typedef typename Intersection<V1, Tail>::type I; typedef typename std::conditional<contains<V1, Head>::value, typename push_back<I, Head>::type, I >::type type; }; template <class V1, class V2> struct Intersection_impl <V1, V2, 0> { typedef vector<> type; }; template <class V1, class V2> struct Intersection { typedef typename Intersection_impl<V1, V2, size<V1>::value * size<V2>::value>::type type; };

So long story short, seems like better days are ahead for C++ meta-programming!