c++ - Declaring member or not depending on template parameter -

is possible declare or not member variable depending on template condition without using dummy empty type?

example:

struct empty{}; struct real_type{};  template<bool condition> struct foo {     typename std::conditional<condition, real_type, empty>::type _member; }; 

you can derive template has specialization:

struct real_type { };  template<bool c> struct foo_base { };  template<> struct foo_base<true> {     real_type _member; };  template<bool condition> struct foo : foo_base<condition> { }; 

as small test:

int main() {     foo<true> t;     t._member.x = 42; // ok      foo<false> f;     f._member.x = 42; // error! no _member exists }