Further const correctness / String by reference passing cleanups (#6571)

There are actually several instances where we pass in read-only
parameters as pass-by-value, where in the case of String() that
is inefficient as it involves copy-constructor/temp string creations.

We can avoid that, similarly to single character string concatenations
done via string literals instead of char literals.

Author: dirkmueller

libraries/ArduinoOTA/ArduinoOTA.cpp

@@ -231,7 +231,7 @@ void ArduinoOTAClass::_onRx(){
       return;
     }
 
-    String challenge = _password + ":" + String(_nonce) + ":" + cnonce;
+    String challenge = _password + ':' + String(_nonce) + ':' + cnonce;
     MD5Builder _challengemd5;
     _challengemd5.begin();
     _challengemd5.add(challenge);

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp

@@ -150,7 +150,7 @@ void HTTPClient::clear()
  * @param https bool
  * @return success bool
  */
-bool HTTPClient::begin(WiFiClient &client, String url) {
+bool HTTPClient::begin(WiFiClient &client, const String& url) {
 #if HTTPCLIENT_1_1_COMPATIBLE
     if(_tcpDeprecated) {
         DEBUG_HTTPCLIENT("[HTTP-Client][begin] mix up of new and deprecated api\n");
@@ -188,7 +188,7 @@ bool HTTPClient::begin(WiFiClient &client, String url) {
  * @param https bool
  * @return success bool
  */
-bool HTTPClient::begin(WiFiClient &client, String host, uint16_t port, String uri, bool https)
+bool HTTPClient::begin(WiFiClient &client, const String& host, uint16_t port, const String& uri, bool https)
 {
 #if HTTPCLIENT_1_1_COMPATIBLE
     if(_tcpDeprecated) {
@@ -281,8 +281,10 @@ bool HTTPClient::begin(String url)
 }
 #endif // HTTPCLIENT_1_1_COMPATIBLE
 
-bool HTTPClient::beginInternal(String url, const char* expectedProtocol)
+bool HTTPClient::beginInternal(const String& __url, const char* expectedProtocol)
 {
+    String url(__url);
+
     DEBUG_HTTPCLIENT("[HTTP-Client][begin] url: %s\n", url.c_str());
     clear();
 
@@ -500,7 +502,7 @@ void HTTPClient::setAuthorization(const char * user, const char * password)
 {
     if(user && password) {
         String auth = user;
-        auth += ":";
+        auth += ':';
         auth += password;
         _base64Authorization = base64::encode(auth);
     }
@@ -533,7 +535,7 @@ void HTTPClient::setTimeout(uint16_t timeout)
  * set the URL to a new value. Handy for following redirects.
  * @param url
  */
-bool HTTPClient::setURL(String url)
+bool HTTPClient::setURL(const String& url)
 {
     // if the new location is only a path then only update the URI
     if (url && url[0] == '/') {
@@ -542,7 +544,7 @@ bool HTTPClient::setURL(String url)
         return true;
     }
 
-    if (!url.startsWith(_protocol + ":")) {
+    if (!url.startsWith(_protocol + ':')) {
         DEBUG_HTTPCLIENT("[HTTP-Client][setURL] new URL not the same protocol, expected '%s', URL: '%s'\n", _protocol.c_str(), url.c_str());
         return false;
     }
@@ -587,16 +589,16 @@ int HTTPClient::GET()
 
 /**
  * sends a post request to the server
- * @param payload uint8_t *
+ * @param payload const uint8_t *
  * @param size size_t
  * @return http code
  */
-int HTTPClient::POST(uint8_t * payload, size_t size)
+int HTTPClient::POST(const uint8_t* payload, size_t size)
 {
     return sendRequest("POST", payload, size);
 }
 
-int HTTPClient::POST(String payload)
+int HTTPClient::POST(const String& payload)
 {
     return POST((uint8_t *) payload.c_str(), payload.length());
 }
@@ -607,26 +609,26 @@ int HTTPClient::POST(String payload)
  * @param size size_t
  * @return http code
  */
-int HTTPClient::PUT(uint8_t * payload, size_t size) {
+int HTTPClient::PUT(const uint8_t* payload, size_t size) {
     return sendRequest("PUT", payload, size);
 }
 
-int HTTPClient::PUT(String payload) {
-    return PUT((uint8_t *) payload.c_str(), payload.length());
+int HTTPClient::PUT(const String& payload) {
+    return PUT((const uint8_t *) payload.c_str(), payload.length());
 }
 
 /**
  * sends a patch request to the server
- * @param payload uint8_t *
+ * @param payload const uint8_t *
  * @param size size_t
  * @return http code
  */
-int HTTPClient::PATCH(uint8_t * payload, size_t size) {
+int HTTPClient::PATCH(const uint8_t * payload, size_t size) {
     return sendRequest("PATCH", payload, size);
 }
 
-int HTTPClient::PATCH(String payload) {
-    return PATCH((uint8_t *) payload.c_str(), payload.length());
+int HTTPClient::PATCH(const String& payload) {
+    return PATCH((const uint8_t *) payload.c_str(), payload.length());
 }
 
 /**
@@ -635,19 +637,19 @@ int HTTPClient::PATCH(String payload) {
  * @param payload String        data for the message body
  * @return
  */
-int HTTPClient::sendRequest(const char * type, String payload)
+int HTTPClient::sendRequest(const char * type, const String& payload)
 {
-    return sendRequest(type, (uint8_t *) payload.c_str(), payload.length());
+    return sendRequest(type, (const uint8_t *) payload.c_str(), payload.length());
 }
 
 /**
  * sendRequest
- * @param type const char *     "GET", "POST", ....
- * @param payload uint8_t *     data for the message body if null not send
- * @param size size_t           size for the message body if 0 not send
+ * @param type const char *           "GET", "POST", ....
+ * @param payload const uint8_t *     data for the message body if null not send
+ * @param size size_t                 size for the message body if 0 not send
  * @return -1 if no info or > 0 when Content-Length is set by server
  */
-int HTTPClient::sendRequest(const char * type, uint8_t * payload, size_t size)
+int HTTPClient::sendRequest(const char * type, const uint8_t * payload, size_t size)
 {
     bool redirect = false;
     int code = 0;
@@ -1212,12 +1214,12 @@ bool HTTPClient::sendHeader(const char * type)
         return false;
     }
 
-    String header = String(type) + " " + (_uri.length() ? _uri : F("/")) + F(" HTTP/1.");
+    String header = String(type) + ' ' + (_uri.length() ? _uri : F("/")) + F(" HTTP/1.");
 
     if(_useHTTP10) {
-        header += "0";
+        header += '0';
     } else {
-        header += "1";
+        header += '1';
     }
 
     header += String(F("\r\nHost: ")) + _host;
@@ -1316,7 +1318,8 @@ int HTTPClient::handleHeaderResponse()
                     if(_currentHeaders[i].key.equalsIgnoreCase(headerName)) {
                         if (_currentHeaders[i].value != "") {
                             // Existing value, append this one with a comma
-                            _currentHeaders[i].value += "," + headerValue;
+                            _currentHeaders[i].value += ',';
+                            _currentHeaders[i].value += headerValue;
                         } else {
                             _currentHeaders[i].value = headerValue;
                         }

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h

@@ -147,8 +147,8 @@ class HTTPClient
  * Since both begin() functions take a reference to client as a parameter, you need to 
  * ensure the client object lives the entire time of the HTTPClient
  */
-    bool begin(WiFiClient &client, String url);
-    bool begin(WiFiClient &client, String host, uint16_t port, String uri = "/", bool https = false);
+    bool begin(WiFiClient &client, const String& url);
+    bool begin(WiFiClient &client, const String& host, uint16_t port, const String& uri = "/", bool https = false);
 
 #if HTTPCLIENT_1_1_COMPATIBLE
     // Plain HTTP connection, unencrypted
@@ -175,20 +175,20 @@ class HTTPClient
     void setTimeout(uint16_t timeout);
     void setFollowRedirects(bool follow);
     void setRedirectLimit(uint16_t limit); // max redirects to follow for a single request
-    bool setURL(String url); // handy for handling redirects
+    bool setURL(const String& url); // handy for handling redirects
     void useHTTP10(bool usehttp10 = true);
 
     /// request handling
     int GET();
-    int POST(uint8_t * payload, size_t size);
-    int POST(String payload);
-    int PUT(uint8_t * payload, size_t size);
-    int PUT(String payload);
-    int PATCH(uint8_t * payload, size_t size);
-    int PATCH(String payload);
-    int sendRequest(const char * type, String payload);
-    int sendRequest(const char * type, uint8_t * payload = NULL, size_t size = 0);
-    int sendRequest(const char * type, Stream * stream, size_t size = 0);
+    int POST(const uint8_t* payload, size_t size);
+    int POST(const String& payload);
+    int PUT(const uint8_t* payload, size_t size);
+    int PUT(const String& payload);
+    int PATCH(const uint8_t* payload, size_t size);
+    int PATCH(const String& payload);
+    int sendRequest(const char* type, const String& payload);
+    int sendRequest(const char* type, const uint8_t* payload = NULL, size_t size = 0);
+    int sendRequest(const char* type, Stream * stream, size_t size = 0);
 
     void addHeader(const String& name, const String& value, bool first = false, bool replace = true);
 
@@ -216,7 +216,7 @@ class HTTPClient
         String value;
     };
 
-    bool beginInternal(String url, const char* expectedProtocol);
+    bool beginInternal(const String& url, const char* expectedProtocol);
     void disconnect(bool preserveClient = false);
     void clear();
     int returnError(int error);

libraries/ESP8266WebServer/src/ESP8266WebServer-impl.h

@@ -252,11 +252,11 @@ void ESP8266WebServerTemplate<ServerType>::requestAuthentication(HTTPAuthMethod
     _srealm = String(realm);
   }
   if(mode == BASIC_AUTH) {
-    sendHeader(String(FPSTR(WWW_Authenticate)), String(F("Basic realm=\"")) + _srealm + String(F("\"")));
+    sendHeader(String(FPSTR(WWW_Authenticate)), String(F("Basic realm=\"")) + _srealm + String('\"'));
   } else {
     _snonce=_getRandomHexString();
     _sopaque=_getRandomHexString();
-    sendHeader(String(FPSTR(WWW_Authenticate)), String(F("Digest realm=\"")) +_srealm + String(F("\", qop=\"auth\", nonce=\"")) + _snonce + String(F("\", opaque=\"")) + _sopaque + String(F("\"")));
+    sendHeader(String(FPSTR(WWW_Authenticate)), String(F("Digest realm=\"")) +_srealm + String(F("\", qop=\"auth\", nonce=\"")) + _snonce + String(F("\", opaque=\"")) + _sopaque + String('\"'));
   }
   using namespace mime;
   send(401, String(FPSTR(mimeTable[html].mimeType)), authFailMsg);
@@ -524,13 +524,13 @@ String ESP8266WebServerTemplate<ServerType>::credentialHash(const String& userna
 {
   MD5Builder md5;
   md5.begin();
-  md5.add(username + ":" + realm + ":" + password);  // md5 of the user:realm:password
+  md5.add(username + ':' + realm + ':' + password);  // md5 of the user:realm:password
   md5.calculate();
   return md5.toString();
 }
 
 template <typename ServerType>
-void ESP8266WebServerTemplate<ServerType>::_streamFileCore(const size_t fileSize, const String & fileName, const String & contentType)
+void ESP8266WebServerTemplate<ServerType>::_streamFileCore(const size_t fileSize, const String &fileName, const String &contentType)
 {
   using namespace mime;
   setContentLength(fileSize);
@@ -544,7 +544,7 @@ void ESP8266WebServerTemplate<ServerType>::_streamFileCore(const size_t fileSize
 
 
 template <typename ServerType>
-const String& ESP8266WebServerTemplate<ServerType>::arg(String name) const {
+const String& ESP8266WebServerTemplate<ServerType>::arg(const String& name) const {
   for (int j = 0; j < _postArgsLen; ++j) {
     if ( _postArgs[j].key == name )
       return _postArgs[j].value;
@@ -590,7 +590,7 @@ bool ESP8266WebServerTemplate<ServerType>::hasArg(const String& name) const {
 
 
 template <typename ServerType>
-const String& ESP8266WebServerTemplate<ServerType>::header(String name) const {
+const String& ESP8266WebServerTemplate<ServerType>::header(const String& name) const {
   for (int i = 0; i < _headerKeysCount; ++i) {
     if (_currentHeaders[i].key.equalsIgnoreCase(name))
       return _currentHeaders[i].value;
@@ -630,7 +630,7 @@ int ESP8266WebServerTemplate<ServerType>::headers() const {
 }
 
 template <typename ServerType>
-bool ESP8266WebServerTemplate<ServerType>::hasHeader(String name) const {
+bool ESP8266WebServerTemplate<ServerType>::hasHeader(const String& name) const {
   for (int i = 0; i < _headerKeysCount; ++i) {
     if ((_currentHeaders[i].key.equalsIgnoreCase(name)) &&  (_currentHeaders[i].value.length() > 0))
       return true;

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -107,17 +107,17 @@ class ESP8266WebServerTemplate
   // Allows setting server options (i.e. SSL keys) by the instantiator
   ServerType &getServer() { return _server; }
 
-  const String& arg(String name) const;    // get request argument value by name
+  const String& arg(const String& name) const;    // get request argument value by name
   const String& arg(int i) const;          // get request argument value by number
   const String& argName(int i) const;      // get request argument name by number
   int args() const;                        // get arguments count
   bool hasArg(const String& name) const;   // check if argument exists
   void collectHeaders(const char* headerKeys[], const size_t headerKeysCount); // set the request headers to collect
-  const String& header(String name) const; // get request header value by name
+  const String& header(const String& name) const; // get request header value by name
   const String& header(int i) const;       // get request header value by number
   const String& headerName(int i) const;   // get request header name by number
   int headers() const;                     // get header count
-  bool hasHeader(String name) const;       // check if header exists
+  bool hasHeader(const String& name) const;       // check if header exists
   const String& hostHeader() const;        // get request host header if available or empty String if not
 
   // send response to the client

libraries/ESP8266WiFiMesh/src/ESP8266WiFiMesh.cpp

@@ -358,7 +358,7 @@ transmission_status_t ESP8266WiFiMesh::exchangeInfo(WiFiClient &currClient)
 {
   verboseModePrint("Transmitting");  // Not storing strings in flash (via F()) to avoid performance impacts when using the string.
 
-  currClient.print(getMessage() + "\r");
+  currClient.print(getMessage() + '\r');
   yield();
 
   if (!waitForClientTransmission(currClient, _stationModeTimeoutMs))
@@ -582,11 +582,11 @@ void ESP8266WiFiMesh::attemptTransmission(const String &message, bool concluding
 
       if(_verboseMode) // Avoid string generation if not required
       {
-        verboseModePrint(String(F("AP acquired: ")) + currentSSID + String(F(", Ch:")) + String(currentWiFiChannel) + " ", false);
+        verboseModePrint(String(F("AP acquired: ")) + currentSSID + String(F(", Ch:")) + String(currentWiFiChannel) + ' ', false);
 
         if(currentNetwork.networkIndex != NETWORK_INFO_DEFAULT_INT)
         {
-          verboseModePrint("(" + String(WiFi.RSSI(currentNetwork.networkIndex)) + String(F("dBm) ")) + 
+          verboseModePrint(String('(') + String(WiFi.RSSI(currentNetwork.networkIndex)) + String(F("dBm) ")) +
                            (WiFi.encryptionType(currentNetwork.networkIndex) == ENC_TYPE_NONE ? String(F("open")) : ""), false);
         }
 
@@ -662,7 +662,7 @@ void ESP8266WiFiMesh::acceptRequest()
       if (_client.connected())
       {
         verboseModePrint("Responding"); // Not storing strings in flash (via F()) to avoid performance impacts when using the string.
-        _client.print(response + "\r");
+        _client.print(response + '\r');
         _client.flush();
         yield();
       }

libraries/ESP8266httpUpdate/src/ESP8266httpUpdate.cpp

@@ -439,7 +439,7 @@ HTTPUpdateResult ESP8266HTTPUpdate::handleUpdate(HTTPClient& http, const String&
  * @param md5 String
  * @return true if Update ok
  */
-bool ESP8266HTTPUpdate::runUpdate(Stream& in, uint32_t size, String md5, int command)
+bool ESP8266HTTPUpdate::runUpdate(Stream& in, uint32_t size, const String& md5, int command)
 {
 
     StreamString error;

libraries/ESP8266httpUpdate/src/ESP8266httpUpdate.h

@@ -130,7 +130,7 @@ class ESP8266HTTPUpdate
 
 protected:
     t_httpUpdate_return handleUpdate(HTTPClient& http, const String& currentVersion, bool spiffs = false);
-    bool runUpdate(Stream& in, uint32_t size, String md5, int command = U_FLASH);
+    bool runUpdate(Stream& in, uint32_t size, const String& md5, int command = U_FLASH);
 
     int _lastError;
     bool _rebootOnUpdate = true;

libraries/ESP8266mDNS/src/ESP8266mDNS_Legacy.cpp

@@ -277,7 +277,7 @@ bool MDNSResponder::addServiceTxt(char *name, char *proto, char *key, char *valu
                 return false;    //max txt record size
             }
             MDNSTxt *newtxt = new MDNSTxt;
-            newtxt->_txt = String(key) + "=" + String(value);
+            newtxt->_txt = String(key) + '=' + String(value);
             newtxt->_next = 0;
             if (servicePtr->_txts == 0)  //no services have been added
             {