c++ - Specializing a function of a template class -

this legal in c++:

template <int n> class {     void bar() {std::cout << n << '\n';} };  template<> void a<2>::bar() {std::cout << "two\n";}  // ok. 

now consider class:

template <int...> struct b;  template <int first, int... rest> struct b<first, rest...> : b<rest...> {     static void foo() {         std::cout << first << ' ';         b<rest...>::foo();       }     static void bar() {/*bunch of code*/}     static void baz() {/*bunch of code*/} };  template <> struct b<> {     static void foo() {}     static void bar() {}     static void baz() {} }; 

then why following illegal (placed after above):

template <int... rest> void b<2, rest...>::foo() {  // illegal.     std::cout << "two ";     b<rest...>::foo(); } 

i don't see why b<2, rest...> incomplete type, error message states. apparently, way achieve want through this?

template <int... rest> struct b<2, rest...> : b<rest...> {     static void foo() {         std::cout << "two ";         b<rest...>::foo();       }     static void bar() {/*same bunch of code above*/}     static void baz() {/*same bunch of code above*/} }; 

thus repeating code in bar() , baz()?

what trying achieve called partial template specialization, , allowed classes, not functions. see example why function template cannot partially specialized?