Fixed value returned by serializeXxx() when writing to a String

extras/tests/Helpers/WString.h

@@ -0,0 +1,39 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#pragma once
+
+#include <string>
+
+// Reproduces Arduino's String class
+class String {
+ public:
+  String& operator+=(char c) {
+    _str += c;
+    return *this;
+  }
+  String& operator+=(int);  // no used, just to add ambiguity
+
+  unsigned char reserve(size_t capacity) {
+    _str.reserve(capacity);
+    return 1;
+  }
+
+  size_t length() const {
+    return _str.size();
+  }
+
+  const char* c_str() const {
+    return _str.c_str();
+  }
+
+ private:
+  std::string _str;
+};
+
+class StringSumHelper;
+
+bool operator==(const std::string& lhs, const ::String& rhs) {
+  return lhs == rhs.c_str();
+}