c++ - How to select the right overloaded function template at compile-time? -

i'm trying understand how select right overloaded function template @ compile-time, compiler giving me hard time. can make work, don't understand going on. let me explain.

i have 2 structs , b below. 1 has special function , other normal.

struct {     void special() { std::cout << "special called.\n"; } };  struct b {     void normal() { std::cout << "normal called.\n"; } }; 

my intension have mechanism, @ compile-time selects right overloaded function template depending on if special function available. have 2 functions run, take struct parameter, can call appropriate function.

template<class func, func f> struct sfinae {};  template <typename u> static void run(u& u, sfinae<void (u::*)(), &u::special>* = 0) {     u.special(); }  template <typename u> static void run(u& u, ...) {     u.normal(); } 

i've tested following, various results:

int main() {     a;     run<a>(a, 0); // works     run<a>(a); // error: ambiguous overloaded function     run(a, 0); // error: has no member normal     run(a); // error: ambiguous overloaded function      b b;     run<b>(b, 0); // works     run<b>(b); // works     run(b, 0); // works     run(b); // works      return 0; } 

i'd use run(a) without argument or <>. there wrong code when not working?

also, i'm interested understand going on here , why deducing things this, need give <a> a not b? don't know standard says , if different between compilers, @ least gcc4.4.4 on linux , gcc 4.0.1 on mac work i've described.

can please shed light on this? thanks!

this here work. sort-of assumes 2 functions normal , special mutually exclusive (i.e. class has 1 of them doesn't have other). i'm sure can adapt purpose. uses boost::enable_if, of course.

#include <iostream> #include <boost/utility/enable_if.hpp>  struct {     void special() { std::cout << "special called.\n"; } };  struct b {     void normal() { std::cout << "normal called.\n"; } };  template<int> struct sfinae { enum { value = true }; };  template <typename u> static typename boost::enable_if<sfinae<sizeof(&u::special)>,void>::type run(u& u) {     u.special(); }  template <typename u> static typename boost::enable_if<sfinae<sizeof(&u::normal)>,void>::type run(u& u) {     u.normal(); }   int main() {     a;     run(a); // works      b b;     run(b); // works      return 0; } 

this works on gcc 4.6.0 on linux.