User can now use a JsonString as a key or a value

src/ArduinoJson/Strings/StlStringAdapter.hpp

@@ -0,0 +1,54 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2018
+// MIT License
+
+#pragma once
+
+#include <string>
+
+namespace ARDUINOJSON_NAMESPACE {
+
+class StlStringAdapter {
+ public:
+  StlStringAdapter(const std::string& str) : _str(&str) {}
+
+  char* save(MemoryPool* pool) const {
+    size_t n = _str->length() + 1;
+    char* dup = pool->allocFrozenString(n);
+    if (dup) memcpy(dup, _str->c_str(), n);
+    return dup;
+  }
+
+  bool isNull() const {
+    return false;
+  }
+
+  bool equals(const char* expected) const {
+    if (!expected) return false;
+    return *_str == expected;
+  }
+
+  const char* data() const {
+    return _str->data();
+  }
+
+  size_t size() const {
+    return _str->size();
+  }
+
+  bool isStatic() const {
+    return false;
+  }
+
+ private:
+  const std::string* _str;
+};
+
+template <>
+struct IsString<std::string> : true_type {};
+
+inline StlStringAdapter adaptString(const std::string& str) {
+  return StlStringAdapter(str);
+}
+
+}  // namespace ARDUINOJSON_NAMESPACE