c++ - Detect operator support with decltype/SFINAE -

a (somewhat) outdated article explores ways use decltype along sfinae detect if type supports operators, such == or <.

here's example code detect if class supports < operator:

template <class t> struct supports_less_than {     static auto less_than_test(const t* t) -> decltype(*t < *t, char(0))     { }      static std::array<char, 2> less_than_test(...) { }      static const bool value = (sizeof(less_than_test((t*)0)) == 1); };  int main() {     std::cout << std::boolalpha << supports_less_than<std::string>::value << endl; } 

this outputs true, since of course std::string supports < operator. however, if try use class doesn't support < operator, compiler error:

error: no match ‘operator<’ in ‘* t < * t’ 

so sfinae not working here. tried on gcc 4.4 , gcc 4.6, , both exhibited same behavior. so, possible use sfinae in manner detect whether type supports expressions?

you need make less_than_test function template, since sfinae stands substitution failure not error , there's no template function can fail selection in code.

template <class t> struct supports_less_than {     template <class u>     static auto less_than_test(const u* u) -> decltype(*u < *u, char(0))     { }      static std::array<char, 2> less_than_test(...) { }      static const bool value = (sizeof(less_than_test((t*)0)) == 1); };  int main() {     std::cout << std::boolalpha << supports_less_than<std::string>::value << endl; }