Re: Derived class & Function pointer

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++
Date:
Mon, 14 Sep 2009 20:25:36 +0200
Message-ID:
<7h7g4jF2s8t9oU1@mid.individual.net>
mast4as wrote:

Hi everyone

I have a problem that many people probably came across before but I
didn't find an answer on the net yet ...

I have a base class Bass and 2 derived class DerivedA & DerivedB. I
can create an new instance of the DeriveA class but then need to
call a method of the DerivedB class to save the data of the base
class into a specific file format. So I thought of using function
pointer but can seem to figure it out:

class Base
{
public:
  float *data;
 Base() { data = new float[10]; }
 ~Base() { delete [] data; }
 virtual void SaveFormat() = 0;
}

class DerivedA : public Base
{
public:
 void SaveFormat() { // save data in file format A }
}

class DerivedB : public Base
{
public:
 void SaveFormat() { // save data in file format B }
}

int main()
{
 Base *derivedA = new Derived A;
 // now I want to save the data hold in derivedA but by using
SaveFormat from DerivedB ???
 ????
 // this is where I am lost... I tried
 void (Base::*SaveFormatPtrFunc)() = &DerivedB::SaveFormat;
 derivedA->*SaveFormatPtrFunc();
 // but that doesn't compile ;-(
}

Does anybody know what's the best way of doing this ?


Not this way for sure. :-)

If you want different ways to save the data, without the format being
tied to the actual type of the objects, why make them members in the
first place?

What about a couple of free functions?

void SaveFormatA(const Base&);
void SaveFormatB(const Base&);

If you still need the virtual functions for a default save format,
these can call the free function of their choice, like

void DerivedA::SaveFormat()
{ SaveFormatA(*this); }

Bo Persson

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