c++ - bool and sizeof conditional template -

i'm testing struct i'm trying use template conditions, i'm having strange compiler errors. here code:

#include <type_traits> #include <string>  template<typename t1, typename t2,     bool same_size = (sizeof(t1)==sizeof(t2))> struct same_size {     typedef typename std::false_type value; }; template<typename t1, typename t2> struct same_size<t1, t2, true> {     typedef typename std::true_type value; };  int main() {     if(same_size<char,unsigned char>::value)     {         printf("yes");     }     system("pause"); } 

i'm compiling in visual studio 2015. these compiler errors get:

1>  main.cpp 1>c:\users\luis\documents\visual studio 2015\projects\stringtype\stringtype\main.cpp(18): error c2059: syntax error: ')' 1>c:\users\luis\documents\visual studio 2015\projects\stringtype\stringtype\main.cpp(19): error c2143: syntax error: missing ';' before '{' 

can shed light going on here?

you having value type, not value. can't use in if condition. best thing can use inheritance , save on typing. so:

#include <type_traits> #include <string>  template<typename t1, typename t2,     bool same_size = (sizeof(t1)==sizeof(t2))> struct same_size : std::false_type { };  template<typename t1, typename t2> struct same_size<t1, t2, true> : std::true_type  { };  int main() {     if(same_size<char,unsigned char>::value)     {         printf("yes");     }     system("pause"); } 

another (better in view) solution suggested @gmannickg:

template<typename t1, typename t2> struct same_size : std::integral_constant<bool, sizeof(t1) == sizeof(t2)> {}; 

the benefits of above are, of course, less typing , less error prone: in first solution, still write same_size<int, int, false>::value , wrong results.

and beauty of second solution still going produce types compatible true_type , false_type, latter typedefs corresponding integral_constant.

on side note, seeing template metaprogramming , printf in same code seeing space shuttle drawn pair of horses.