output_container.h

/*
 * Written by Wu Yongwei <wuyongwei AT gmail DOT com>.
 *
 * Using this file requires a C++17-compliant compiler.
 *
 * This is free and unencumbered software released into the public domain.
 *
 * Anyone is free to copy, modify, publish, use, compile, sell, or
 * distribute this software, either in source code form or as a compiled
 * binary, for any purpose, commercial or non-commercial, and by any
 * means.
 *
 * In jurisdictions that recognize copyright laws, the author or authors
 * of this software dedicate any and all copyright interest in the
 * software to the public domain. We make this dedication for the benefit
 * of the public at large and to the detriment of our heirs and
 * successors. We intend this dedication to be an overt act of
 * relinquishment in perpetuity of all present and future rights to this
 * software under copyright law.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 * For more information, please refer to <http://unlicense.org>
 *
 */

#ifndef OUTPUT_CONTAINER_H
#define OUTPUT_CONTAINER_H

#include <ostream>     // std::ostream
#include <type_traits> // std::false_type/true_type/decay_t/is_same_v
#include <utility>     // std::declval/pair

// Type trait to detect std::pair
template <typename T> struct is_pair : std::false_type {};
template <typename T, typename U>
struct is_pair<std::pair<T, U>> : std::true_type {};
template <typename T> inline constexpr bool is_pair_v = is_pair<T>::value;

// Type trait to detect whether an output function already exists
template <typename T> struct has_output_function {
  template <class U>
  static auto output(U *ptr)
      -> decltype(std::declval<std::ostream &>() << *ptr, std::true_type());
  template <class U> static std::false_type output(...);
  static constexpr bool value = decltype(output<T>(nullptr))::value;
};
template <typename T>
inline constexpr bool has_output_function_v = has_output_function<T>::value;
/* NB: Visual Studio 2017 (or below) may have problems with
 *     has_output_function_v<T>: you should then use
 *     has_output_function<T>::value instead, or upgrade to
 *     Visual Studio 2019. */

// Output function for std::pair
template <typename T, typename U>
std::ostream &operator<<(std::ostream &os, const std::pair<T, U> &pr);

// Element output function for containers that define a key_type and
// have its value type as std::pair
template <typename T, typename Cont>
auto output_element(std::ostream &os, const T &element, const Cont &,
                    const std::true_type)
    -> decltype(std::declval<typename Cont::key_type>(), os);
// Element output function for other containers
template <typename T, typename Cont>
auto output_element(std::ostream &os, const T &element, const Cont &, ...)
    -> decltype(os);

// Main output function, enabled only if no output function already exists
template <typename T, typename = std::enable_if_t<!has_output_function_v<T>>>
auto operator<<(std::ostream &os, const T &container)
    -> decltype(container.begin(), container.end(), os) {
  using std::decay_t;
  using std::is_same_v;

  using element_type = decay_t<decltype(*container.begin())>;
  constexpr bool is_char_v = is_same_v<element_type, char>;
  if constexpr (!is_char_v) {
    os << "{ ";
  }
  if (!container.empty()) {
    auto end = container.end();
    bool on_first_element = true;
    for (auto it = container.begin(); it != end; ++it) {
      if constexpr (is_char_v) {
        if (*it == '\0') {
          break;
        }
      }
      if constexpr (!is_char_v) {
        if (!on_first_element) {
          os << ", ";
        } else {
          on_first_element = false;
        }
      }
      output_element(os, *it, container, is_pair<element_type>());
    }
  }
  if constexpr (!is_char_v) {
    os << " }";
  }
  return os;
}

template <typename T, typename Cont>
auto output_element(std::ostream &os, const T &element, const Cont &,
                    const std::true_type)
    -> decltype(std::declval<typename Cont::key_type>(), os) {
  os << element.first << " => " << element.second;
  return os;
}

template <typename T, typename Cont>
auto output_element(std::ostream &os, const T &element, const Cont &, ...)
    -> decltype(os) {
  os << element;
  return os;
}

template <typename T, typename U>
std::ostream &operator<<(std::ostream &os, const std::pair<T, U> &pr) {
  os << '(' << pr.first << ", " << pr.second << ')';
  return os;
}

#endif // OUTPUT_CONTAINER_H