Using SFINAE to check for global operator<<?

Question

I want to have several overloaded, global to_string() functions that take some type T and convert it to its string representation. For the general

Answer 1

The initializer of value on line 48 is not in a context where SFINAE works. Try moving the expression to the function declaration.

#include 

struct sfinae_base {
  typedef char yes[1];
  typedef char no[2];
};

template
struct has_insertion_operator : sfinae_base {

  // this may quietly fail:
  template static yes& test(
      size_t (*n)[ sizeof( std::cout << * static_cast(0) ) ] );

  // "..." provides fallback in case above fails
  template static no& test(...);

  static bool const value = sizeof( test( NULL ) ) == sizeof( yes );
};

However, I have to question the amount of sophistication that is going into this. I see non-orthogonal mechanisms that will grind against each other (to_string vs. operator<<) and I hear poor assumptions getting tossed around (for example that operator<< is global vs a member, although the code as implemented looks OK in that regard).