support bazel complie this project and format code.

learn_class/modern_cpp_30/container1/output_container.h

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+/*
+ * 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