serializeXxx() sets std::string and String instead of appending

CHANGELOG.md

@@ -26,3 +26,4 @@ HEAD
 * Remove `JsonDocument::garbageCollect()`
 * Add `deserializeJson(JsonVariant, ...)` and `deserializeMsgPack(JsonVariant, ...)` (#1226)
 * Call `shrinkToFit()` in `deserializeJson()` and `deserializeMsgPack()`
+* `serializeJson()` and `serializeMsgPack()` replace the content of `std::string` and `String` instead of appending to it

extras/tests/Helpers/api/String.h

@@ -9,8 +9,11 @@
 // Reproduces Arduino's String class
 class String {
  public:
-  String() : maxCapacity_(1024) {}
-  explicit String(const char* s) : str_(s), maxCapacity_(1024) {}
+  String() = default;
+  String(const char* s) {
+    if (s)
+      str_.assign(s);
+  }
 
   void limitCapacityTo(size_t maxCapacity) {
     maxCapacity_ = maxCapacity;
@@ -33,7 +36,10 @@ class String {
   }
 
   String& operator=(const char* s) {
+    if (s)
       str_.assign(s);
+    else
+      str_.clear();
     return *this;
   }
 
@@ -59,7 +65,7 @@ class String {
 
  private:
   std::string str_;
-  size_t maxCapacity_;
+  size_t maxCapacity_ = 1024;
 };
 
 class StringSumHelper : public ::String {};

extras/tests/JsonSerializer/std_string.cpp

@@ -12,14 +12,14 @@ TEST_CASE("serialize JsonArray to std::string") {
   array.add(2);
 
   SECTION("serializeJson()") {
-    std::string json;
+    std::string json = "erase me";
     serializeJson(array, json);
 
     REQUIRE("[4,2]" == json);
   }
 
   SECTION("serializeJsonPretty") {
-    std::string json;
+    std::string json = "erase me";
     serializeJsonPretty(array, json);
 
     REQUIRE("[\r\n  4,\r\n  2\r\n]" == json);
@@ -32,14 +32,14 @@ TEST_CASE("serialize JsonObject to std::string") {
   obj["key"] = "value";
 
   SECTION("object") {
-    std::string json;
+    std::string json = "erase me";
     serializeJson(doc, json);
 
     REQUIRE("{\"key\":\"value\"}" == json);
   }
 
   SECTION("serializeJsonPretty") {
-    std::string json;
+    std::string json = "erase me";
     serializeJsonPretty(doc, json);
 
     REQUIRE("{\r\n  \"key\": \"value\"\r\n}" == json);
@@ -50,7 +50,7 @@ TEST_CASE("serialize an std::string containing a NUL") {
   JsonDocument doc;
   doc.set(std::string("hello\0world", 11));
 
-  std::string json;
+  std::string json = "erase me";
   serializeJson(doc, json);
   CHECK("\"hello\\u0000world\"" == json);
 }

extras/tests/Misc/StringWriter.cpp

@@ -141,7 +141,7 @@ TEST_CASE("Writer<custom_string>") {
 TEST_CASE("serializeJson(doc, String)") {
   JsonDocument doc;
   doc["hello"] = "world";
-  ::String output;
+  ::String output = "erase me";
 
   SECTION("sufficient capacity") {
     serializeJson(doc, output);

src/ArduinoJson/Serialization/Writers/ArduinoStringWriter.hpp

@@ -13,8 +13,10 @@ class Writer<::String, void> {
   static const size_t bufferCapacity = ARDUINOJSON_STRING_BUFFER_SIZE;
 
  public:
-  explicit Writer(::String& str) : destination_(&str) {
-    size_ = 0;
+  explicit Writer(::String& str) : destination_(&str), size_(0) {
+    // clear the string but don't use "" to avoid useless allocation
+    // https://cpp4arduino.com/2018/11/21/eight-tips-to-use-the-string-class-efficiently.html
+    str = static_cast<const char*>(0);
   }
 
   ~Writer() {

src/ArduinoJson/Serialization/Writers/StdStringWriter.hpp

@@ -24,7 +24,9 @@ template <typename TDestination>
 class Writer<TDestination,
              typename enable_if<is_std_string<TDestination>::value>::type> {
  public:
-  Writer(TDestination& str) : str_(&str) {}
+  Writer(TDestination& str) : str_(&str) {
+    str.clear();
+  }
 
   size_t write(uint8_t c) {
     str_->push_back(static_cast<char>(c));