dev-cpp-users

[Dev-C++] newbie dynamic_cast question

[<tc...@gm...>]

Hi all,

I have a dynamic_cast problem and I am not comming forward since then in 
my project. Therefore I decided too find some help here in ths ML.

OK,

I actually want to implement the iterator design Pattern in order to 
iterate through abstract Elements of abstract CollectionObjects. 
Therefore I have a class Element and CollectionObject.

Now I have written some concrete implementing classes Directory and File 
that are supposed to be both Elements, with the supplement that 
Directory is also a CollectionObject.

So I have sth. like

class Element{...}

class FileSystem : public Element { }

class File : public FileSystem {...}

class Directory : public CollectionObject, public FileSystem { ... }

Now I decided to include some Filters, Iteated elements should be 
matched by Patterns (abstract class). I have created for this one a 
PatternExtension concrete class. It all looks lie this:

class Pattern {
...
public:
	virtual bool match( Element *p_Element ) = 0;
}


class PatternExtension : public Pattern
{
	...
	virtual bool match( Element *p_Element )
	{
		//here starts the problem:
		//writing this instruction is causes an Error from the 				//compiler. 
Therefore I have been looking in my doc.
		//and I am convince to be doing it properly....

	    File *l_File;

	    if( l_File = dynamic_cast<File *>( p_Element ) != NULL )
		{
		...
		}
	/*
		Compiler error message:

PatternExtension.cpp:19: cannot dynamic_cast `p_Element' (of type `class 
fs::Element *') to type `class fs::File *'
make[1]: *** [PatternExtension.o] Fehler 1
make[1]: Verlassen des Verzeichnisses Verzeichnis 
»/work/Projects/C++/Project1/fs«
make: *** [all] Fehler 2
	*/
	}
}

Why then?

I hope I wasn't too long and that I made myself clear enough. I would be 
pleased for any hint, tip to help me comming forward.

Thanx

--
Eric Tchepannou

Re: [Dev-C++] newbie dynamic_cast question

[Sebastien M. <seb...@te...>]

Hi!

Ok, I just check documentation, it seems you can't use a dynamic cast
in your case because File is not a base type for Element (of course,
since Element is a base class for File :)

So maybe you should use static_cast, or directly a standard C cast
(personnaly, I found all the cast operator useless, even if it's more
safe, it's too boring too use them :) and I never got a problem with
cast, bugs are generally elsewhere :)

I can give you a good documentation, but it's in french, so if you can
read french, here it is:
http://www.developpez.com/c/megacours/x4009.html#AEN4058

Happy coding,
Sebastien.

On Sun, Dec 08, 2002 at 07:51:09AM +0100, tc...@gm... wrote:
> Hi all,
>=20
> I have a dynamic_cast problem and I am not comming forward since then i=
n=20
> my project. Therefore I decided too find some help here in ths ML.
>=20
> OK,
>=20
> I actually want to implement the iterator design Pattern in order to=20
> iterate through abstract Elements of abstract CollectionObjects.=20
> Therefore I have a class Element and CollectionObject.
>=20
> Now I have written some concrete implementing classes Directory and Fil=
e=20
> that are supposed to be both Elements, with the supplement that=20
> Directory is also a CollectionObject.
>=20
> So I have sth. like
>=20
> class Element{...}
>=20
> class FileSystem : public Element { }
>=20
> class File : public FileSystem {...}
>=20
> class Directory : public CollectionObject, public FileSystem { ... }
>=20
> Now I decided to include some Filters, Iteated elements should be=20
> matched by Patterns (abstract class). I have created for this one a=20
> PatternExtension concrete class. It all looks lie this:
>=20
> class Pattern {
> ...
> public:
> 	virtual bool match( Element *p_Element ) =3D 0;
> }
>=20
>=20
> class PatternExtension : public Pattern
> {
> 	...
> 	virtual bool match( Element *p_Element )
> 	{
> 		//here starts the problem:
> 		//writing this instruction is causes an Error from the 	=20
> 		//compiler. Therefore I have been looking in my doc.
> 		//and I am convince to be doing it properly....
>=20
> 	    File *l_File;
>=20
> 	    if( l_File =3D dynamic_cast<File *>( p_Element ) !=3D NULL )
> 		{
> 		...
> 		}
> 	/*
> 		Compiler error message:
>=20
> PatternExtension.cpp:19: cannot dynamic_cast `p_Element' (of type `clas=
s=20
> fs::Element *') to type `class fs::File *'
> make[1]: *** [PatternExtension.o] Fehler 1
> make[1]: Verlassen des Verzeichnisses Verzeichnis=20
> =BB/work/Projects/C++/Project1/fs=AB
> make: *** [all] Fehler 2
> 	*/
> 	}
> }
>=20
> Why then?
>=20
> I hope I wasn't too long and that I made myself clear enough. I would b=
e=20
> pleased for any hint, tip to help me comming forward.
>=20
> Thanx
>=20
> --
> Eric Tchepannou
>=20
>=20
>=20
> -------------------------------------------------------
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> Welcome to geek heaven.
> http://thinkgeek.com/sf
> _______________________________________________
> Dev-cpp-users mailing list
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>=20

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