c++ - How to dispatch to templated call operator using SFINAE -

i have function takes templated callable argument, , passes index it. in situations, index passed statically (i'm working tuples). thought should possible passing callable object templated call operator , using sfinae.

at first, looks like:

struct {     template< size_t >     void operator()( int x )     {         cout << "a " << x << " " << << endl;     } };  struct b {     void operator()( int x, int )     {         cout << "b " << x << " " << << endl;     } };   template<     typename f,     size_t = 0 > inline void call( int x, f & fn ) {     fn( x, ); }   int main() {     a;             b b;      call( 2, b );     call< b, 3 >( 2, b );      call( 1, ); // no match call '(a) (int&, long unsigned int)'      return 0; } 

so try overload call function , select right invocation using sfinae:

template<     typename f,     size_t = 0 > inline typename std::enable_if< /* i've tried kinds of things here */ >::type call( int x, f & fn ) {     fn< >( x ); } 

but can't figure out type traits detect whether f callable 1 template parameter , int argument. i've been referencing this article , this one having trouble adapting them use case. ideas? possible without modifying call site?

struct {   template< std::size_t >   void operator()( int x, std::integral_constant<std::size_t, i> ) {     cout << "a " << x << " " << << endl;   } }; 

use instead. standard sfinae tests work, no passing template nontype arguments.

in c++14 compiler can std::get<i>. in c++11 compiler can std::get<i> or std::get<decltype(i)::value>.

passing template non-type arguments sucks. avoid them.