Re: Derived class & Function pointer
mast4as wrote:
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.
Doesn't sound right. Sounds like a design pulled by the ears to do
something that isn't really its purpose.
> 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 ?
The ONLY way of doing this is to implement your file-saving separately
from the data altogether. See "Visitor" pattern.
Neither 'DerivedA' nor 'DerivedB' should actually be derived from Base
if all you need to do is to save 'Base' in some specific format. You
need two classes 'SaverA' and 'SaverB', possibly derived from 'Saver',
and call those (possibly polymorphically) from 'Base' itself. The
pointer to the 'Saver' should be set by the derived class object:
class Saver
{
public:
virtual void save(class Base*);
};
class Base
{
...
virtual void saveSelf() { if (saver) saver->save(this); }
protected:
Saver* saver;
};
class SaverA : public Saver
{
void save(Base*) { ... } // format A
};
class SaverB : public Saver
{
void save(Base*) { ... } // format B
};
class DerivedA : public Base
{
public:
DerivedA() { saver = new SaverA; }
};
class DerivedB : public Base
{
public:
DerivedB() { saver = new SaverB; }
};
class CrossSaver : public Base
{
CrossSaver() { saver = 0; } // no saver
void saveSelf() {
SaverA sa;
saver = &sa; Base::saveSelf();
SaverB sb;
saver = &sb; Base::saveSelf();
}
};
V
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