sfinae.cpp

#include <iostream>
#include <map>
#include <string>
#include <type_traits>
#include <utility>

using std::declval;
// check that two expression make sense:
// a) m[key]
// b) m.find(key) != m.end()
template <
    class Map, class Key,
    class SFINAE = std::void_t<decltype(declval<Map>()[declval<Key>()]),
                               decltype(declval<Map>().find(declval<Key>()) !=
                                        declval<Map>().end())>>
bool has_key(const Map &map, const Key &key) {
  return map.find(key) != map.end();
}

class KindOfAMap {
public:
  double operator[](const std::string &key) { return 42.0; }
};

bool has_key(const KindOfAMap &map, const std::string &key) { return true; }

int main() {

  std::map<std::string, double> m;
  m["foo"] = 32.0;

  std::cout << has_key(m, "foo") << "\n";

  auto kinda_map = KindOfAMap();
  std::cout << has_key(kinda_map, "foo") << "\n";

  return 0;
}