Re: Member function pointer to member function of another class

From:

Praetorian <ashish.sadanandan@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 2 Apr 2008 10:54:30 -0700 (PDT)

Message-ID:

<7e8eb55d-0fa9-43f1-9760-e7d424f39238@e6g2000prf.googlegroups.com>

On Apr 2, 10:14 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

Praetorian wrote:

Hi,
I'm having huge syntax problem dealing withfunctionpointers. Here's
what I'm doing:

I have 2 classes, ClassA and ClassB. ClassA owns an instance of
ClassB. What I want to do is pass afunctionpointerto amember
functionof ClassB as a parameter to amemberfunctionof ClassA. I
don't want ClassA'smemberfunctionto call ClassB'sfunctiondirectly
because ClassA can be used in 2 ways, one of which is a case in which
it does not create an instance of ClassB; hence by passing afunction
pointerI can handle that case easily.

This is the code I have:

classClassB {
public:
   BOOL GetFlagX(void) { return m_bFlagX;};
private:
   BOOL m_bFlagX;
};

classClassA {
public:
   typedef BOOL (ClassA::*FPAbortFlag)(void);
   FPAbortFlag m_fpAbort;

protected:
   ClassB *m_pClassB;
   BOOL ClassAMemFcn(FPAbortFlag fpAbort);
};

In this case ClassA has new'd an instance of ClassB. What I want to do
is have ClassA'sfunctionpointer'm_fpAbort' point to ClassB's
'GetFlagX'function. Can't seem to figure out the syntax for this.

I did get the following to compile but it wasn't pointing to the
memberfunctionassociated with ClassA's instance of ClassB:

m_fpAbort = (FPAbortFlag)(&(ClassB::GetFlagX));

There should be no need for the cast or for the parentheses.

m_fpAbort = &ClassB::GetFlagX;

If this doesn't work for you, post the code so we can take a closer
look (even copy-and-paste it locally and compile). See FAQ 5.8.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

Victor,
That didn't work; I get the following compiler error:

error C2440: '=' : cannot convert from 'BOOL (__thiscall
CBootLoaderDlg::* )(void)' to 'CFlashDownload::FPFlashAbort'
        Types pointed to are unrelated; conversion requires
reinterpret_cast, C-style cast or function-style cast

I tried the reinterpret_cast suggestion as follows:

m_fpAbortFlash = reinterpret_cast <FPFlashAbort>
&CBootLoaderDlg::GetDlgCancelledFlag;

That compiles but the function never returns true; even though I can
breakpoint and verify that the flag is set. It looks like it isn't
pointing to the correct instance of CBootLoaderDlg.

Here are snippets of the actual code:

In FlashDownload.h

#include "BootLoader.h"
#include "BootLoaderDlg.h"

class CFlashDownload
{
public:
  /* Constructor/Destructor */
  CFlashDownload();
  ~CFlashDownload();

  void FlashDownloadWithUI(void);
private:
  CBootLoaderDlg *m_pBootDlg;

  typedef BOOL (CFlashDownload::*FPFlashAbort)();
  FPFlashAbort m_fpAbortFlash;

  BOOL Flash2812InternalRam(FPFlashAbort fpAbortFlash);
};

In BootloaderDlg.h

class CBootLoaderDlg : public CDialog
{
    DECLARE_DYNAMIC(CBootLoaderDlg)

public:
    BOOL GetDlgCancelledFlag(void) { return m_bDlgCancelled; };
private:
    BOOL m_bDlgCancelled;
};

In FlashDownload.cpp

void CFlashDownload::FlashDownloadWithUI(void)
{
  if( m_pBootDlg != NULL ) {
    AfxMessageBox( _T("Recursive call to FlashDownloadWithUI()
detected") );
    AfxThrowUserException();
  }

  m_pBootDlg = new CBootLoaderDlg();
  m_pBootDlg->Create( IDD_MAIN_DLG, AfxGetMainWnd() );

  m_fpAbortFlash = &CBootLoaderDlg::GetDlgCancelledFlag;
  /* Call flash download function */
  Flash2812InternalRam(m_fpAbortFlash);

  m_pBootDlg->DestroyWindow();
  delete m_pBootDlg;
}

CBootLoaderDlg sets m_bDlgCancelled when the dialog is cancelled
(within its OnCancel()) method).

Thanks for your help, I really appreciate it. Sorry for this silly
question but you mentioned I should look at section 5.8 of the C++ FAQ
but how do I get to the FAQ?

Ashish.