Problem compiling C++/Arduino app that is using my derived class

I made a derived class called 'WebServer_My2KNJ.h' (see attached) from a working class called WebServer_My2.h (see attached) and I got the following error when I tried compiling the example program 'Web_HelloWorld.ino'.

The compile Error message: (or see the attached file 'ErrorMsg.txt'):

Web_HelloWorld.ino: In function 'void setup()':
Web_HelloWorld:57: error: invalid conversion from 'void (*)(WebServer_My2KNJ&, WebServer_My2::ConnectionType, char*, bool)' to 'void (*)(WebServer_My2&, WebServer_My2::ConnectionType, char*, bool)'
Web_HelloWorld:57: error: initializing argument 1 of 'void WebServer_My2::setDefaultCommand(void (*)(WebServer_My2&, WebServer_My2::ConnectionType, char*, bool))'
Web_HelloWorld:61: error: invalid conversion from 'void (*)(WebServer_My2KNJ&, WebServer_My2::ConnectionType, char*, bool)' to 'void (*)(WebServer_My2&, WebServer_My2::ConnectionType, char*, bool)'
Web_HelloWorld:61: error: initializing argument 2 of 'void WebServer_My2::addCommand(const char*, void (*)(WebServer_My2&, WebServer_My2::ConnectionType, char*, bool))'

The line 'webserver.setDefaultCommand(&helloCmd);' in the file 'Web_HelloWorld.ino' (see attached) is highlighted in the Arduino IDE as being responsible for the error.

(i) I am a Newbie to C++, so bear with me.
(ii) All files used except 'WebServer_My2KNJ.h' are basically the same found at ''.
(iii) In the parent class 'WebServer_My2.h', all I did was to modify 'WebServer.h' by changing 'private:' to 'protected:', and the class name.
(iv) 'Web_HelloWorld.ino' compiles and runs as expected if I use 'WebServer_My2.h' instead of 'WebServer_My2KNJ.h'.
(v) The reason I am going to this trouble, is that I want to later add more functionality to this class. Hence to keep things cleaner and simpler, it is best I have my code in a derived class.

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Consider this a shot in the dark (as I can't test it), but since you didn't change any behaviour of the implementation, a cast could fix that, e.g.

void setup()
  /* initialize the Ethernet adapter */
  Ethernet.begin(mac, ip);

  /* setup our default command that will be run when the user accesses
* the root page on the server */

  /* run the same command if you try to load /index.html, a common
* default page name */
  webserver.addCommand("index.html", static_cast<WebServer_My2>(&helloCmd));

  /* start the webserver */

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error: invalid conversion from 'void (*)(WebServer_My2KNJ&, WebServer_My2::ConnectionType, char*, bool)' to 'void (*)(WebServer_My2&, WebServer_My2::ConnectionType, char*, bool)'

the error means that the function pointer used in line 57 of Web_HelloWorld has not the prototype that was required. the first argument of your function was WebServer_My2KNJ& while the required argument type was WebServer_My2&. a function pointer cannot use inheritance on its arguments. so even if WebServer_My2KNJ is derived from class WebServer_My2 the function pointer argument could not be mapped and resolve that.

there are possibly 3 ways out:

- you may try the cast jkr has posted. he tries to cast the function pointer you provided to the required type.  it may work if WebServer_My2KNJ is derived directly from class WebServer_My2. it will not work if there is no inheritance between the classes or if multiple inheritance is involved.

- you may define the function as required. that means you change the first argument type from WebServer_My2KNJ& to WebServer_My2&. of course if you do so, and want to use the enhancements that your derived class has over the baseclass,  you have to virtually call into your derived class to using those enhancements in the function, cause the baseclass does not have access to those enhancements. note, you also could do a dynamic_cast on the argument but a virtual call always is the better choice before a cast.

- you may change the required function pointer type to taking a reference variable of your class rather than the WebServer_My2&. that could only work if you have the implementation (source code) of the function called _and_ if you want to replace the WebServer_My2 by your class.

the 3rd way is the only way if your class is not derived from WebServer_My2.

Hope4UAuthor Commented:
[1] The above threw new errors... see the attached.

[2] I have manage to gotten somewhere with the code with the help of a guru... it might be the solution: see  I would have liked it that I wouldn't have to use the '#include WebServer_My2.h' in the main app 'Web-HelloWorld.ino'.  This stuff is complicated with trying to cast a pointer to a function... I was looking at overloading the functions 'webserver.setDefaultCommand(&helloCmd);', etc. the type def of 'Command(...)' to become:
typedef void Command(WebServer_My2KNJ &server, ConnectionType type,
                       char *url_tail, bool tail_complete);

but ended up with problems trying to cast the '*cmd' function pointer:

void WebServer_My2KNJ::setDefaultCommand(Command *cmd)
  m_defaultCmd = static_cast<WebServer_My2:Command>(cmd);  //<-- Got errors here
I would have liked it that I wouldn't have to use the '#include WebServer_My2.h' in the main app 'Web-HelloWorld.ino'.

if WebServer_My2KNJ is derived from WebServer_My2 then you need to include the WebServer_My2.h cause you need the class definition of your base class.

if WebServer_My2KNJ is a replacement of WebServer_My2, then  WebServer_My2KNJ.h should be used instead of WebServer_My2.h. additionally, you then need to replace all occurrences of WebServer_My2 in the WebServer_My2KNJ.h and all other source files.

m_defaultCmd = static_cast<WebServer_My2:Command>(cmd);  //<-- Got errors here

between WebServer_My2 and Command should be two colon characters :: . the member function should be a static function if using the typedef you got from the "guru" both in your class and in WebServer_My2. if it is not a static member function in WebServer_My2 then the function pointer actually is a pointer to member function what is one of the most complex and less flexible c++ concepts where you should keep off.

note, as told above, casting is the worse solution to your issue, especially if your goal is to replace the WebServer_My2 rather than to enhance it.

the simplest solution for you is to replace the WebServer_My2 by WebServer_My2KNJ at all places, thus removing all references to  WebServer_My2.
(and do no cast). instead define your overload of WebServer_My2KNJ::MyCommand as
void WebServer_My2KNJ::MyCommand(WebServer_My2 &server, ConnectionType type,
                     char *url_tail, bool tail_complete);

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the second best solution is to derive from WebServer_My2 but don't change the typedef in
WebServer_My2.h and don't add a new typedef in your header.

with that, the statement

void WebServer_My2::setDefaultCmd(Command * cmd)
     m_defaultCmd = cmd; 

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compiles and you would call such function from everywhere in WebServer_My2KNJ member functions by


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note, the this-> can be omitted in all non-static member functions of your class if you derive public from WebServer_My2.

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