Add string view [C++] (#4730)

* Add view() method on flatbuffers::String, to return a string_view type
if support for std::string_view (or alternately
std::experimental::string_view) is found

* Move detection/definition of FLATBUFFERS_STRING_VIEW to base.h, use the
macro (if it is defined) as the argument type for an overload of CreateString

* Rename String::view() to String::string_view() and use the existing c_str() method for the data pointer

* Add and explain minimum C++ standard version checks for FLATBUFFERS_STRING_VIEW implementations

* Updated preprocessor indenting for FLATBUFFERS_STRING_VIEW

* Convert FLATBUFFERS_STRING_VIEW macro to typedef in flatbuffers:: namespace, and boolean feature toggle macro FLATBUFFERS_HAS_STRING_VIEW

* Prepend flatbuffers:: namespace to disambiguate flatbuffers::string_view typedef from String::string_view()

* clang-format as-she-is-spoke for FLATBUFFERS_HAS_STRING_VIEW

include/flatbuffers/base.h

@@ -153,6 +153,28 @@
   #pragma warning(disable: 4127) // C4127: conditional expression is constant
 #endif
 
+#ifndef FLATBUFFERS_HAS_STRING_VIEW
+  // Only provide flatbuffers::string_view if __has_include can be used
+  // to detect a header that provides an implementation
+  #if defined(__has_include)
+    // Check for std::string_view (in c++17)
+    #if __has_include(<string_view>) && (__cplusplus > 201402)
+      #include <string_view>
+      namespace flatbuffers {
+        typedef std::string_view string_view;
+      }
+      #define FLATBUFFERS_HAS_STRING_VIEW 1
+    // Check for std::experimental::string_view (in c++14, compiler-dependent)
+    #elif __has_include(<experimental/string_view>) && (__cplusplus > 201103)
+      #include <experimental/string_view>
+      namespace flatbuffers {
+        typedef std::experimental::string_view string_view;
+      }
+      #define FLATBUFFERS_HAS_STRING_VIEW 1
+    #endif
+  #endif // __has_include
+#endif // !FLATBUFFERS_HAS_STRING_VIEW
+
 /// @endcond
 
 /// @file

include/flatbuffers/flatbuffers.h

@@ -344,6 +344,14 @@ struct String : public Vector<char> {
   const char *c_str() const { return reinterpret_cast<const char *>(Data()); }
   std::string str() const { return std::string(c_str(), Length()); }
 
+  // clang-format off
+  #ifdef FLATBUFFERS_HAS_STRING_VIEW
+  flatbuffers::string_view string_view() const {
+    return flatbuffers::string_view(c_str(), Length());
+  }
+  #endif // FLATBUFFERS_HAS_STRING_VIEW
+  // clang-format on
+
   bool operator<(const String &o) const {
     return strcmp(c_str(), o.c_str()) < 0;
   }
@@ -1077,6 +1085,17 @@ class FlatBufferBuilder {
     return CreateString(str.c_str(), str.length());
   }
 
+  // clang-format off
+  #ifdef FLATBUFFERS_HAS_STRING_VIEW
+  /// @brief Store a string in the buffer, which can contain any binary data.
+  /// @param[in] str A const string_view to copy in to the buffer.
+  /// @return Returns the offset in the buffer where the string starts.
+  Offset<String> CreateString(flatbuffers::string_view str) {
+    return CreateString(str.data(), str.size());
+  }
+  #endif // FLATBUFFERS_HAS_STRING_VIEW
+  // clang-format on
+
   /// @brief Store a string in the buffer, which can contain any binary data.
   /// @param[in] str A const pointer to a `String` struct to add to the buffer.
   /// @return Returns the offset in the buffer where the string starts