Pointer to non static member functions within the class

Hi,

after spending a lot of time on the following problem, without any luck, I first wanted to ask if my problem is solvable:

Code:

---

typedef void (*fp)(int, const char*);
 
class A
{
  public:
    A()
    {
      fp x;
      x = &A::staticMemberFunc; // works
      x = &A::memberFunc; // this doesn't, but I want it to...
      // (translated) error message:  
      // error C2440: '=':  'void (__thiscall A::* )(int,const char *)' could not be converted to 'fp'
    }
 
  private:
    void memberFunc(int, const char*) {}
    static void staticMemberFunc(int, const char*) {}
};
 
 
int main(int argc, char* argv[])
{
  return 0;
}

---

If it is, some hints are highly appreciated.


Thanks,

Lykurg

P.s.: The typedef is part of a library so I can't change it.

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Added after 21 minutes:

Damn! Not again ;) Found the solution right after posting...

Code:

---

class A
{
  public:
    A()
    {
      foo = &A::memberFunc;
      fp x;
      x = (fp) &foo;
    }
 
    void (A::*foo)(int, const char*);
    void memberFunc(int, const char*) {}
};

---

---

Added after 18 minutes:

Ehm, :crying:, no compiler error but I get a crash when using x.

Quote:

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Ehm, , no compiler error but I get a crash when using x.

---

As long as you don't touch the parameters, it should work (but will be quite useless). First parameter of a class member method is an address of calling object ('this'), so casting it like that will cause a mess, look at this one:

Code:

---

#include <iostream>
 
typedef void (*fp)(int, int);
 
class A
{
  public:
    void method( int x, int y){
		std::cout << (int)this << " " << x;
	}
};
 
typedef void (A::*fpa)(int,int); 
 
int main(int argc, char* argv[])
{   
  fpa x = &A::method;
 
  fp f;
  f = (fp)(x);
  f(44,55);
 
  return 0;
}

---

output is : 44 55
Arguments got "shifted", because an address of calling object is expected as first parameter ( so in fact it is void method( A * this, int, int ) ). I think it's clear enough that any use of 'this' (referencing) in such method will cause a crash, as 'this' is not valid address of calling object ( trying to call object's method without an object :) ). Last parameter passed will probably contain some garbage values.

Is this some kind of callback mechanism and you want to register object's method ?

Hi,

thanks for your comment, I see a little bit clearer now. But also your example don't compile (msvc) or crashes with mingw.

My original "problem" is to make a library call to astyle (because I had rearrange my plugin last time and want to release a new version). The idea was to encapsulate all astyle stuff in a single class, with simple returns the formatted source code and it should be used in parallel to speed up things for future. I can make a work around with a static error function and a mutex and grab the error string inside formatedText(), but I it would be nicer to directly use a member function (if possible).

Code:

---

QString ASInstance::formatedText(const QString& text)
{
  m_error = false;
  QByteArray baIn = text.toUtf8();
  const char* textOut = AStyleMain(baIn.constData(),
                                   m_options.constData(),
                                   /* MAGIC goes here, must be fpError */,
                                   &ArtisticStyle::Internal::ASInstance::memoryAlloc);
  QString formattedText = QString::fromUtf8(textOut);
  delete [] textOut;
  return formattedText;
}
 
void ASInstance::errorHandler(int number, const char* message)
{
  m_error = true;
  m_errorNumber = number;
  m_errorMessage = QString(message);
}

---

and from astyle.h

Code:

---

typedef void (STDCALL* fpError)(int, const char*);      // pointer to callback error handler
extern "C" EXPORT char* STDCALL AStyleMain(const char*, const char*, fpError, fpAlloc);

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Quote:

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But also your example don't compile (msvc) or crashes with mingw.

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I don't have msvc compiler, but it worked with g++ 4.5.2, nevermind anyway.
If you want to use member method, the only thing I can think of in that case is to pass static class method pointer as callback and use member method of static instance inside, but it has obvious limitations. I think it's not possible to use member method directly, because of the "this" issue.
Maybe someone else has encountered that kind of problem and can help you more.