C++ classes help

allomeen · 12-21-2005, 10:56 AM

Hello everyone,
I have this problem and I don't know how to solve it. I have two classes Timer and Power. The Timer class have a member function that accept a void *(*ptr)() ;a pointer to a function and then start a thread and does some things. the Power class instantiate a Timer object and calls that function passing an address of one of its member function.

class Timer
{
public:
void setFunction(void *(*fun)());
}

class Power
{
public:
Timer t;
private:
Power();
void * update();
}

Power::Power()
{
//...code...//
t.setFunction(update);
//...code...//
}
Power::update()
{
//...code...//
}

The compiler gives me this error:
"error: no matching function for call to `Timer::setFunction(<unkown type>)'
note: candidates are: void Timer::setFunction(void*(*)())"

Can anybody tell me what wrong here and why doesn't the compiler know the type of update?

Thanks,
Alaa G

dmail · 12-21-2005, 11:11 AM

To use a class function as a function ptr the function has to be static to the class, but this also means it can not call any members of the class which are not static. The way I normaly get around this is to use a friend function helper, something like this:

Code:

int helper(void* data)
{
   //start the thread
   ....
}

class Foo
{
  public:
   ...
  friend int helper(void* data);
  private:
  ....
};

just as a side note is this correct?

Code:

class Power
{
public:
Timer t;
private:
Power();
void * update();
};

or did you mean

Code:

class Power
{
private:
Timer t;
public:
Power();
void * update();
};

tuxdev · 12-21-2005, 11:11 AM

try declaring this function as

Code:

void setFunction(void *(*)() fun);

it may help to create a typedef for your function pointer type.

allomeen · 12-21-2005, 11:19 AM

Thanks guys. I just declared update to be static and it worded.

ta0kira · 12-26-2005, 04:47 PM

Does the 'update' function need to do anything to the 'Power' object? If it's static, it might as well be outside of the class. You can make it work the other way if you change 'setFunction' to

Code:

//Put this before Timer
class Power;

//Within the Timer class
void setFunction(Power *power, void *(Power:: *fun)());

This will allow you to tie the Timer directly to the Power object. Then change the 'setFunction' call to

Code:

t.setFunction(this, &Power::update);

When you call the function from Timer, call it like this

Code:

(power->*fun)();

That will make sure the Power object that called 'setFunction' will get the function call (i.e. you can update information within the Power object in the 'update' function.) If you leave it static, none of the Power objects will get the call; it will be like calling a global function.

You should always use '&' when giving a function pointer argument just out of good form, and you should also qualify even static functions with the class name, just in case.
ta0kira

PS dmail: OP is using a private function pointer, however OP provides it from within the class so there is no need for friends or making the function public. I do agree that 't' should be private as well, however.