gistfile1.cpp

// ref: https://plus.google.com/u/0/111121989384217966500/posts/LjSZjmKGpjj

// Tested with gcc 4.6.3
// $ g++ -std=c++0x THISFILE && a.out

#include <cassert>
#include <type_traits>

using namespace std;

template <typename T>
struct get_impl;

template <>
struct get_impl<int>
{
  static int call()
  {
    return 0;
  }
};

template <>
struct get_impl<double>
{
  static double call()
  {
    return 1.0;
  }
};

struct auto_getter
{
  template <typename T>
  operator T () const
  {
    return get_impl<T>::call();
  }
};

template <typename T>
T get()
{
  return get_impl<T>::call();
}

inline auto_getter get()
{
  return auto_getter{};
}

int f(int)
{
  return 0;
}
int f(double)
{
  return 1;
}

void example()
{
  {
    int a = get();
    assert(a == 0);
    double b = get();
    assert(b == 1.0);
  }

  {
    auto a = get<int>();
    assert(a == 0);
    auto b = get<double>();
    assert(b == 1.0);
  }

  assert(f(get<int>()) == 0);
  assert(f(get<double>()) == 1);

  // <dont_do_this>
  auto c = get();
  bool c_is_auto_getter = is_same<decltype(c), auto_getter>::value;
  assert(c_is_auto_getter);
  int d = c;
  assert(d == 0);
  double e = c;
  assert(e == 1.0);
  // </dont_do_this>

  //string x = get();      // compile-time error
  //auto y = get<string>();// compile-time error
}

int main()
{
  example();
  return 0;
}