c++ - Is it possible to write a template to check for a function's existence? -

is possible write template changes behavior depending on if member function defined on class?

here's simple example of want write:

template<class t> std::string optionaltostring(t* obj) {     if (function_exists(t->tostring))         return obj->tostring();     else         return "tostring not defined"; } 

so, if class t has tostring() defined, uses it; otherwise, doesn't. magical part don't know how "function_exists" part.

yes, sfinae can check if given class provide method. here's working code:

#include <iostream>  struct hello {     int helloworld() { return 0; } };  struct generic {};      // sfinae test template <typename t> class has_helloworld {     typedef char one;     typedef long two;      template <typename c> static 1 test( typeof(&c::helloworld) ) ;     template <typename c> static 2 test(...);      public:     enum { value = sizeof(test<t>(0)) == sizeof(char) }; };  int main(int argc, char *argv[]) {     std::cout << has_helloworld<hello>::value << std::endl;     std::cout << has_helloworld<generic>::value << std::endl;     return 0; } 

i've tested linux , gcc 4.1/4.3. don't know if it's portable other platforms running different compilers.