recursive variadic template to print out the contents of a parameter pack

Question

How is it possible to create a recursive variadic template to print out the contents of a paramater pack? I am trying with this, but it fails to compile:

template <typename First, typename ...Args> std::string type_name () {     return std::string(typeid(First).name()) + " " + type_name<Args...>(); } std::string type_name () {     return ""; } 

How shall I end the recursion?

Answer 1

There's actually a very elegant way to end the recursion:

template <typename Last> std::string type_name () {     return std::string(typeid(Last).name()); }  template <typename First, typename Second, typename ...Rest> std::string type_name () {     return std::string(typeid(First).name()) + " " + type_name<Second, Rest...>(); } 

I initially tried template <typename Last> and template <typename First, typename ...Rest> but that was considered ambiguous (Rest can be zero elements). This question then showed me the definitive solution: Compilation Error on Recursive Variadic Template Function

---

Note, to avoid a bit of code duplication, you could also do:

template <typename Last> std::string type_name () {     return std::string(typeid(Last).name()); }  template <typename First, typename Second, typename ...Rest> std::string type_name () {     return type_name<First>() + " " + type_name<Second, Rest...>(); } 

Answer 2

You need to use partial specialisation to end the recursion, but since you can't partially specialise free functions in C++, you need to create an implementation class with a static member function.

template <typename... Args> struct Impl;  template <typename First, typename... Args> struct Impl<First, Args...> {   static std::string name()   {     return std::string(typeid(First).name()) + " " + Impl<Args...>::name();   } };  template <> struct Impl<> {   static std::string name()   {     return "";   } };  template <typename... Args> std::string type_name() {     return Impl<Args...>::name(); }  int main() {   std::cout << type_name<int, bool, char, double>() << std::endl; // "i b c d"   return 0; } 

That first declaration of Impl is just a workaround for a shortcoming in g++ 4.6 (and below). It won't be necessary once it implements variadic templates correctly.

Check it out in action at ideone.com