Question about function pointers

[MrDoomMaster]

I'm trying to create a typedef which can create a function pointer to a function which returns void and has no parameters, such as the example delcaration you see below:

void TestFunc(void);

I've tried the following:

typedef void (*_FuncPtr)(void);

So, if I do this:

_FuncPtr FunctionPtr = TestFunc;

FunctionPtr();

The compiler returns an error saying that FunctionPtr does not evaluate to a function taking 0 arguments.

How would I appropriately create a function pointer such as this?

Thanks!

[MrViggy]

I think the proper way to use a function pointer, to call the function that it is pointing to, is:

(*FunctionPtr)();

Viggy

[MrDoomMaster]

Viggy,

I tried your suggested calling method, and my compiler returned the following error:

error C2171: '*' : illegal on operands of type 'thu::_ThreadProc'


Here is more code for your reference:

namespace thu
{
        /* Typedefs */
    typedef void (*_ThreadProc)(void);
};

[Kheun]

MrDoomMaster, I can't see anything wrong with your code, your example actually works on VC6. Probably, it has something wrong with your definition of TestFunc().

#include <stdio.h>

void TestFunc(void)
{
	printf("TestFunc\n");
}

typedef void (*_FuncPtr)(void);

int main()
{
	_FuncPtr FunctionPtr = TestFunc;
	FunctionPtr();

	return 0;
}

[MrDoomMaster]

Well, maybe it will help if I post all of my code.

It's not much, it's just a class I've created which makes creating threads a little simpler. I haven't had the chance to test it yet due to this compiler error, but perhaps it will give you a better chance of finding what I'm doing wrong. This class is contained in a Header file and CPP file:

First, my Header File:

//=========================================================================
//=========================================================================
//          Multithreading Utilities Library (Thread_Utils)
//
//      Author: Robert Dailey (MrDoomMaster)
//
//=========================================================================
//=========================================================================

#ifndef                 _THREAD_UTILS_
#define                 _THREAD_UTILS_

//--Includes---------------------------------------------------------------
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include                <windows.h>

//-------------------------------------------------------------------------



//=============================================================================================
// Namespaces
//=============================================================================================
/*
||
*/

namespace thu
{
        /* Typedefs */
    typedef void (*_ThreadProc)(void);
};

//=============================================================================================



//=============================================================================================
// Classes
//=============================================================================================
/*
|| This class provides the entire functionality of this library. It supports safe
|| multithreading.
*/
class Thread_Utils
{
    private:
        //--Variables-----------------------------------------------------------
        //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        bool                    ThreadRunning                               ;
        DWORD                   ThreadID                                    ;
        HANDLE                  ThreadHandle                                ;
        HANDLE                  StopHandle                                  ;
        HANDLE                  WaitHandle                                  ;
        thu::_ThreadProc        ThreadCode                                  ;
        //----------------------------------------------------------------------


        //--Prototypes----------------------------------------------------------
        //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        static                  DWORD WINAPI EntryPoint(LPVOID pThis);
        //----------------------------------------------------------------------

    public:
        //--Prototypes----------------------------------------------------------
        //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            /*--Constructors/Destructors--*/
        Thread_Utils(thu::_ThreadProc ThreadFunction);
        virtual ~Thread_Utils();

            /*--Functions--*/
        void                    StartThread(void);
        void                    StopThread(void);
        //virtual void            ThreadCode(void);
        //----------------------------------------------------------------------
};

//=============================================================================================

#endif

And my CPP file:

//=========================================================================
//=========================================================================
//          Multithreading Utilities Library (Thread_Utils)
//
//      Author: Robert Dailey (MrDoomMaster)
//
//=========================================================================
//=========================================================================

#include                "Thread_Utils.h"


//===========================================================================================
// Function Definitions
//===========================================================================================
/*
|| // TODO: Provide descriptions for all functions
*/
DWORD WINAPI Thread_Utils::EntryPoint(LPVOID pThis)
{
    while(true)
    {
        if(::WaitForSingleObject(((Thread_Utils*)pThis)->StopHandle, 0) == WAIT_OBJECT_0)
        {
            ::SetEvent(((Thread_Utils*)pThis)->WaitHandle);
            ::ExitThread(0);
        }

        ThreadCode();
    }
}

//===========================================================================================
/*
||
*/
Thread_Utils::Thread_Utils(thu::_ThreadProc ThreadFunction)
{
    ThreadHandle    = NULL;
    StopHandle      = NULL;
    WaitHandle      = NULL;
    ThreadID        = 0;
    ThreadCode      = ThreadFunction;
}

//===========================================================================================
/*
||
*/
Thread_Utils::~Thread_Utils(void)
{
    if(ThreadHandle)
        StopThread();
}

//===========================================================================================
/*
||
*/
void Thread_Utils::StartThread(void)
{
    StopHandle      = ::CreateEvent(NULL, TRUE, FALSE, NULL);
    WaitHandle      = ::CreateEvent(NULL, TRUE, FALSE, NULL);
    ThreadHandle    = ::CreateThread(NULL, 0, EntryPoint, this, 0, &ThreadID);
}

//===========================================================================================
/*
||
*/
void Thread_Utils::StopThread(void)
{
    ::SetEvent(StopHandle);

    ::WaitForSingleObject(WaitHandle, INFINITE);

    ::CloseHandle(StopHandle);
    ::CloseHandle(WaitHandle);
    ::CloseHandle(ThreadHandle);

    StopHandle      = NULL;
    WaitHandle      = NULL;
    ThreadHandle    = NULL;
}

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

//===========================================================================================

I have bolded parts of my code which relate to the issue at hand, for your convenience. Thank you for your time, I hope my code isn't too rough to navigate!

[Kheun]

You have made a small mistake in your static function. Your static function doesn't have the this pointer for invoking the function pointer. You need to invoke the it using pThis.

DWORD WINAPI Thread_Utils::EntryPoint(LPVOID pThis)
{
    while(true)
    {
        if(::WaitForSingleObject(((Thread_Utils*)pThis)->StopHandle, 0) == WAIT_OBJECT_0)
        {
            ::SetEvent(((Thread_Utils*)pThis)->WaitHandle);
            ::ExitThread(0);
        }

        ((Thread_Utils*)pThis)->ThreadCode(); // this pointer isn't available in the static funciton.
    }
}

[MrDoomMaster]

Wow, thanks! I always miss stupid little things like that

[Andreas Masur]

error C2171: '*' : illegal on operands of type 'thu::_ThreadProc'

Well...that is simply because 'ThreadProc()' is a class member function. There is a difference between function pointers to global and to class member functions...

[HuangRG]

Your TestFunc is class member function? if yes, write to following:
static void TestFunc()
{
.......
}

[_uj]

I think the proper way to use a function pointer, to call the function that it is pointing to, is:

(*FunctionPtr)();

Viggy

Also have a look at the "smart" pointers at Boost and Loki.