Classes/Threading: (classname::)(void*)' does not match `void (*)(void*)'

[Adam01]

Hi,
I've been working on a message system using threading:

class console {
    private:
    HANDLE OutThread;
    char * out;
    bool Loopflag;
    void MessageLoop(void *){
        for (;!Loopflag;){
        cout << out;
        out = "";
        Sleep(1000);
        }
    }
    public:
    void write(char* message){ out=message;}
    void start(){ OutThread = (HANDLE)_beginthread(MessageLoop,0,this);}
}console;

But I get this error:
error: argument of type `void (console::)(void*)' does not match `void (*)(void*)'

I tried different ways of getting around this problem, like using MessageLoop() as a static function, but it didnt like using the 'out' char.

Can someone give me help with this please.


-Adam

[Savage]

You can't take address of member function without some type of not recommended fiddling,but you can make your MessageLoop function static.

[Adam01]

I tried that, and I get the following error:

18|error: invalid use of member `console::Loopflag' in static member function|
20|error: from this location|
17|error: invalid use of member `console::out' in static member function|
21|error: from this location|
17|error: invalid use of member `console::out' in static member function|
22|error: from this location|
||=== Build finished: 6 errors, 0 warnings ===|

[Savage]

I tried that, and I get the following error:

18|error: invalid use of member `console::Loopflag' in static member function|
20|error: from this location|
17|error: invalid use of member `console::out' in static member function|
21|error: from this location|
17|error: invalid use of member `console::out' in static member function|
22|error: from this location|
||=== Build finished: 6 errors, 0 warnings ===|

It does that because static functions do not have this pointer.
Can this class be a singleton class?

[newb16]

I tried that, and I get the following error:

It's common techinique - declare MyClass* me variable and cast from void * to it, like

static void Class::method(void*arg)
{
  Class *me = reinterpret_cast<Class*>(arg);
  me->privatemember = 0;
  ...
}
 

[Adam01]

Sorry, what do you mean by singleton class?

[Savage]

Sorry, what do you mean by singleton class?

Singleton patern

,or you can only turn your class members to be static,thus making them accessibly without this pointer.(Consequences )

[Savage]

It's common techinique - declare MyClass* me variable and cast from void * to it, like

static void Class::method(void*arg)
{
  Class *me = reinterpret_cast<Class*>(arg);
  me->privatemember = 0;
  ...
}
 

Then he cannot use that pointer for any additional data he needs.

[Adam01]

I couldnt get that working anyway.

I made all the functions and variables static:

class console {
    private:
    static HANDLE OutThread;
    static char * out;
    static bool Loopflag;
    static void MessageLoop(void *){
        while(!Loopflag){
        cout << out;
        out = "";
        Sleep(1000);
        }
    }
    public:
    static void write(char* message){ out=message;}
    static void start(){ OutThread = (HANDLE)_beginthread(MessageLoop,0,0);}
}console;

And then another error occurs:
obj\Debug\main.o||In function `_ZSt17__verify_groupingPKcjRKSs':|
C:\Program Files\CodeBlocks\MinGW\bin\..\lib\gcc\mingw32\3.4. 5\..\..\..\..\include\c++\3.4.5\bits\locale_facets .tcc|2498|undefined reference to `console::out'|
||=== Build finished: 1 errors, 0 warnings ===|



Savage, would it be ok for you to correct the problem code?
So I could study the method you used.

[edwardrsmith]

First off, I can't tell if you are trying to compile it as c or c++. If it is c then I believe cout will not work (I program under linux so it may be different under windows). The other problem could be that you forgot to include the apropriate header file.

Hope that helped,
Edward

[Savage]

I tried this on VC++ 9 Express and it works.
Can you give us at which line is compiler complaining?
Have you pre-initialized static variables?

[Adam01]

Im using code::blocks, GNU GCC compiler, and the error is in locale_facets.tcc on line 2498.

Would it be ok to correct the the code on my first post?

Pre-initalized, as in declared them before the class? no.

[Savage]

Im using code::blocks, GNU GCC compiler, and the error is in locale_facets.tcc on line 2498.

Would it be ok to correct the the code on my first post?

Pre-initalized, as in declared them before the class? no.

Because static members are not tied to any object of specific class,but rather are tied to the type of object(class it self),you need to initialize them before any type of use.

This would do the tick:

class console
{
  //...static members and other stuff
 
};
 
 
bool console::Loopflag=false;//or true if that's what you want.
char *console::out=0;
HANDLE console::OutThread=0;

[Adam01]

Doing that, as it is after the class, the MessageLoop() function calls to undefined Loopflag and out.

[Savage]

Doing that, as it is after the class, the MessageLoop() function calls to undefined Loopflag and out.

That's how it looks,but if you try stepping into the MessageLoop() with debugger you would see that it runs,it runs simultaneously with other threads and that's why it exits.Try putting a pause at end of your main,and change out="" to something else and you'll see that it works as expected.(That is if you don't have a debugger )

[Adam01]

I've never used step into before, and dont know how.

[Savage]

I've never used step into before, and dont know how.

Do you know how to set a breakpoint?
What does your compiler documentation has to say about it?

[Adam01]

I can set breakpoints, and I can see step into and step out under the debug menu.
I have not read the documentation.

[Savage]

I can set breakpoints, and I can see step into and step out under the debug menu.
I have not read the documentation.

OK,set the breakpoint here:

static void MessageLoop(void *)
    {
 
        while(!Loopflag)
        {
          cout << out;
          out = "";
          Sleep(1000);
        }
    }

And you will see that it runs forever(unless you set LoopFlag to true),and that's just what it is supposed to do.

You can verify this by changing out="" to something else and see that new data gets printed every second.

[Adam01]

I tried that, but it still lists 'out' as an error.

[Savage]

I tried that, but it still lists 'out' as an error.

Oh sorry my bad,I've thought it runs(as it should be).
Have you tried to specify the scope of those variables(e.g instead out,console::out)?

[Adam01]

I tried that, with static, and it still does not work.
Wasnt there a pointer method of this?

[Savage]

I tried that, with static, and it still does not work.
Wasnt there a pointer method of this?

This is the code I have,and in which I'm 100% sure it works.
Create a new project and lets see whether error lies somewhere else:

#include <iostream>
#include <windows.h>
#include <process.h>
 
using namespace std;
 
 
class console {
    private:
    //..............(Members)
    static void MessageLoop(void *){
 
        while(!Loopflag){
 
            cout << out;
            Sleep(1000);
        }
    }
    public:
   //.........(Methods)
 
}console;
 
bool console::Loopflag=false;
char *console::out=0;
HANDLE console::OutThread=0;
 
 
int main()
{
    console.start();
    console.write("\nHello,World!!");
    system("pause");
    return 0;
}

[Adam01]

Yay!!!, thanks a lot savage.

[Savage]

Yay!!!, thanks a lot savage.

Isn't it this the same code you have had?

[Adam01]

Ohh, I see now, when I added the declarations after the class, I removed the declarations from the class. Which triggered the error.
Thanks for your help savage.