Error running FirebaseDemo on nodeMCU

Question

So I followed the guide on GitHub to run this demo which compiles and runs. It does warn of some deprecations and such in the firebase libraries :

In member function 'const String& FirebaseArduino::error()' warning: returning reference to temporary [-Wreturn-local-addr] return error_.message().c_str();

It is failing to update my firebase database and the firebase error message is not being displayed, seems to be because of the above error. But this does make it complicated to debug...

Can someone please help me get around this?

Appreciate any feedback or pro tips :)

UPDATE

So found an issue on GitHub that solved connecting to the database. Fingerprint update apparently... https://github.com/FirebaseExtended/firebase-arduino/issues/369 but still would like to solve the firebase library error so that everyone can get legible error messages back when using it.

Answer 1

It does warn of some deprecations and such in the firebase libraries :

In member function 'const String& FirebaseArduino::error()' warning: returning reference to temporary [-Wreturn-local-addr] return error_.message().c_str();

i also get some depreciation like

/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp: In member function 'bool FirebaseObject::isNullString(const String&) const':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp:67:57: warning: 'const char* ArduinoJson::Internals::JsonVariantCasts<TImpl>::asString() const [with TImpl = ArduinoJson::JsonVariant]' is deprecated (declared at /home/praz/Arduino/libraries/ArduinoJson-5.13.1/src/ArduinoJson/Deserialization/../JsonVariantCasts.hpp:28): use as<char*>() instead [-Wdeprecated-declarations]
   return variant.is<const char *>() && variant.asString() == NULL;
                                                         ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp: In member function 'ArduinoJson::JsonVariant FirebaseObject::getJsonVariant(const String&) const':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseObject.cpp:87:26: warning: 'ArduinoJson::JsonObject& ArduinoJson::Internals::JsonVariantCasts<TImpl>::asObject() const [with TImpl = ArduinoJson::JsonVariant]' is deprecated (declared at /home/praz/Arduino/libraries/ArduinoJson-5.13.1/src/ArduinoJson/Deserialization/../JsonVariantCasts.hpp:23): use as<JsonObject>() instead [-Wdeprecated-declarations]
     json = json.asObject().get<JsonVariant>(start);
                          ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/throw_out_of_range.cpp:6:28: warning: unused parameter 'str' [-Wunused-parameter]
 void __attribute__((weak)) __throw_out_of_range(const char* str) {
                            ^
In file included from /home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseCloudMessaging.cpp:1:0:
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseCloudMessaging.h:82:79: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
   const void AddToJson(const FirebaseCloudMessage& message, JsonObject& json) const;
                                                                               ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseCloudMessaging.cpp:80:60: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
     const FirebaseCloudMessage& message, JsonObject& json) const {
                                                            ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp: In member function 'int FirebaseRequest::sendRequest(const string&, const string&, char*, const string&, const string&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp:76:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp: In member function 'void FirebaseStream::startStreaming(const string&, const string&, const string&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/Firebase.cpp:89:48: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   analyzeError("STREAM", status, path_with_auth);
                                                ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'String FirebaseArduino::push(const String&, const ArduinoJson::JsonVariant&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:65:79: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   int status = req_.get()->sendRequest(host_, auth_, "POST", path.c_str(), buf);
                                                                               ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:65:7: warning: unused variable 'status' [-Wunused-variable]
   int status = req_.get()->sendRequest(host_, auth_, "POST", path.c_str(), buf);
       ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'void FirebaseArduino::set(const String&, const ArduinoJson::JsonVariant&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:94:65: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   req_.get()->sendRequest(host_, auth_, "PUT", path.c_str(), buf);
                                                                 ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'void FirebaseArduino::getRequest(const String&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:101:60: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   req_.get()->sendRequest(host_, auth_, "GET", path.c_str());
                                                            ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'virtual void FirebaseArduino::remove(const String&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:147:63: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   req_.get()->sendRequest(host_, auth_, "DELETE", path.c_str());
                                                               ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp: In member function 'virtual const String& FirebaseArduino::error()':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseArduino.cpp:201:33: warning: returning reference to temporary [-Wreturn-local-addr]
   return error_.message().c_str();
                                 ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp: In member function 'virtual void FirebaseHttpClientEsp8266::begin(const string&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp:47:50: warning: 'bool HTTPClient::begin(String, String)' is deprecated (declared at /home/praz/.arduino15/packages/esp8266/hardware/esp8266/2.4.2/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:156) [-Wdeprecated-declarations]
     http_.begin(url.c_str(), kFirebaseFingerprint);
                                                  ^
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp: In member function 'virtual void FirebaseHttpClientEsp8266::begin(const string&, const string&)':
/home/praz/Arduino/libraries/firebase-arduino-master/src/FirebaseHttpClient_Esp8266.cpp:51:80: warning: 'bool HTTPClient::begin(String, uint16_t, String, String)' is deprecated (declared at /home/praz/.arduino15/packages/esp8266/hardware/esp8266/2.4.2/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h:157) [-Wdeprecated-declarations]
     http_.begin(host.c_str(), kFirebasePort, path.c_str(), kFirebaseFingerprint);
                                                                                ^
Compiling library "ESP8266HTTPClient"

to fixed that, go to Preferences in Compiler Warnings choose as Default. it ignored all warning in depreciation, don't forget to update your library to get Firebase Fingerprint has changed on 2 Aug 2018.

Uploaded your sketch again, your should be get data like

Answer 2

It is quite right to tell you that. The offending code is here:

const String& FirebaseArduino::error() {
  return error_.message().c_str();
}

In that, it calls the message() function on the error_ object to get a std::string object, which it then gets the C string data from with c_str(). It then creates (due to the return type) a temporary String object and copies the C string data into it.

A reference to that temporary String object is then returned and the String is then destroyed immediately. That's bad. The returned reference now points to nothing in particular.

It wouldn't surprise me if there were other places where that kind of thing happens. Which would explain why things aren't necessarily working right.

You should raise an issue on the Github issues page for that project with the author.