c++ - Variadic Templates - different types of expansion -

andrei alexandrescu gave excellent talk entitled: variadic templates funadic.

he presents following 3 expansions subltey different:

template <class... ts> void fun( ts... vs ) {     gun( a<ts...>::hun(vs)...);     gun( a<ts...>::hun(vs...));     gun( a<ts>::hun(vs)...); } 

he explains:

call 1: expands ts instatiation of class a, calls hun(vs) expands parameters again when passing them gun

call 2: expands ts , vs separately

call 3: expnads in lock step, ie: expand argument 1 of ts , argument 1 of vs expand argument 2 of ts , argument 2 of vs expand argument n of ts , argument n of vs

other discussion on variadic templates seem cover simple variadic class templates , variadic functions such typesafe printf etc. unsure how these different types of expansion effect code , each type useful.

does have examples demonstrate application of each type of expansion?

#include <iostream> #include <memory> #include <typeinfo> #include <cstdlib> #include <cxxabi.h>  template <typename t> std::unique_ptr<char, void(*)(void*)> type_name() {     return std::unique_ptr<char, void(*)(void*)>            (                 __cxxabiv1::__cxa_demangle(typeid(t).name(), nullptr,                                            nullptr, nullptr),                 std::free            ); }  void display() {}  template <class t> void display() {     std::cout << type_name<t>().get() << ' '; }  template <class t, class t2, class ...tail> void display() {     std::cout << type_name<t>().get() << ' ';     display<t2, tail...>(); }  template <class... ts> struct {     template <class... us>         static         int         hun(us... us)         {             std::cout << "a<";             display<ts...>();             std::cout << ">::hun(";             display<us...>();             std::cout << ")\n";             return 0;         } };  template <class ...t> void gun(t...) {}  template <class... ts> void fun( ts... vs ) {     std::cout << "gun( a<ts...>::hun(vs)...);\n";     gun( a<ts...>::hun(vs)...);     std::cout << "\ngun( a<ts...>::hun(vs...));\n";     gun( a<ts...>::hun(vs...));     std::cout << "\ngun( a<ts>::hun(vs)...);\n";     gun( a<ts>::hun(vs)...); }  int main() {     fun(1, 'a', 2.3); } 

output:

gun( a<ts...>::hun(vs)...); a<int char double >::hun(int ) a<int char double >::hun(char ) a<int char double >::hun(double )  gun( a<ts...>::hun(vs...)); a<int char double >::hun(int char double )  gun( a<ts>::hun(vs)...); a<int >::hun(int ) a<char >::hun(char ) a<double >::hun(double )