Pointer to non static member functions within the class

[Lykurg]

Hi,

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

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;
}

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;
}

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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.

---

Added after 21 minutes:

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

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

class A
{
  public:
    A()
    {
      foo = &A::memberFunc;
      fp x;
      x = (fp) &foo;
    }

    void (A::*foo)(int, const char*);
    void memberFunc(int, const char*) {}
};

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---

Added after 18 minutes:

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

[stampede]

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:

#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;
}

#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;
}

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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 ?

[Lykurg]

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).

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);
}

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);
}

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and from astyle.h

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

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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[stampede]

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

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.