How to check if another object is my superclass from a function in aneven higher superclass?

[bart van deenen]

Hi

I have a pile of objects all derived from one baseclass, and I want to
have a generic function that an object can use to see if another
object is its superclass. Here's an example that does not work:

#include <stdio.h>
#include <typeinfo>

class A {
public:
void check(A *other) {
__typeof__(this) x = dynamic_cast<__typeof__(other)>(other);
if ( x ) printf("ok\n");
else printf("NULL\n");
}
};

class B : public A { };
class C: public B { };
class D: public A { };

int main()
{
D d;
A a;
C c;
printf("c.check(&a)\n");
c.check(&a);
printf("c.check(&d)\n");
c.check(&d);
}

g++ -frtti -Wall -o main main.cpp

c.check(&a)
ok
c.check(&d)
ok
but C does not inherit from D!

I now know more about the function of the __typeof__ operator, so I
understand why it doesn't work. But how would you do this? Is there an
elegant way?

 

[bart van deenen]

One solution I have is that fact that I'm using Qt QObject derived
objects, so I can use the metaObject() information that is provided by
the meta object precompiler. Like this:

if ( this->inherits( other->metaObject()->className()) )
...

which works perfectly well, but I just wonder if there is no more
generic C++ solution for this.

Bart

 

[Kai-Uwe Bux]

Hi

I have a pile of objects all derived from one baseclass, and I want to
have a generic function that an object can use to see if another
object is its superclass.

[snip: something using __typeof__]

I now know more about the function of the __typeof__ operator, so I
understand why it doesn't work.

First, __typeof__ does not exists in standard C++. There is a typeof
operator, though.

But how would you do this? Is there an elegant way?

Well, for starters, there is the use of virtual functions:

class Base {
protected:

virtual
bool do_check ( Base * ptr ) {
return ( true );
}

public:

bool subclass ( Base * ptr ) {
return do_check( ptr );
}

bool superclass ( Base * ptr ) {
return ptr->do_check( this );
}

};

class D1 : public Base {

bool do_check ( Base * ptr ) {
return ( dynamic_cast< D1* >( ptr ) );
}

};

class D2 : public D1 {

bool do_check ( Base * ptr ) {
return ( dynamic_cast< D2* >( ptr ) );
}

};

class D3 : public Base {

bool do_check ( Base * ptr ) {
return ( dynamic_cast< D3* >( ptr ) );
}

};


#include <iostream>
#include <iomanip>

#define CHECK(expr) std::cout << #expr << " : " \
<< std::boolalpha << expr << "\n";

int main ( void ) {
Base b;
D1 d1;
D2 d2;
D3 d3;
CHECK( b.subclass( &b ) );
CHECK( b.subclass( &d1 ) );
CHECK( b.subclass( &d2 ) );
CHECK( b.subclass( &d3 ) );
CHECK( d1.subclass( &b ) );
CHECK( d1.subclass( &d1 ) );
CHECK( d1.subclass( &d2 ) );
CHECK( d1.subclass( &d3 ) );
CHECK( d2.subclass( &b ) );
CHECK( d2.subclass( &d1 ) );
CHECK( d2.subclass( &d2 ) );
CHECK( d2.subclass( &d3 ) );
CHECK( d3.subclass( &b ) );
CHECK( d3.subclass( &d1 ) );
CHECK( d3.subclass( &d2 ) );
CHECK( d3.subclass( &d3 ) );

CHECK( b.superclass( &b ) );
CHECK( b.superclass( &d1 ) );
CHECK( b.superclass( &d2 ) );
CHECK( b.superclass( &d3 ) );
CHECK( d1.superclass( &b ) );
CHECK( d1.superclass( &d1 ) );
CHECK( d1.superclass( &d2 ) );
CHECK( d1.superclass( &d3 ) );
CHECK( d2.superclass( &b ) );
CHECK( d2.superclass( &d1 ) );
CHECK( d2.superclass( &d2 ) );
CHECK( d2.superclass( &d3 ) );
CHECK( d3.superclass( &b ) );
CHECK( d3.superclass( &d1 ) );
CHECK( d3.superclass( &d2 ) );
CHECK( d3.superclass( &d3 ) );
}

The next observation is that do_check() looks more or less alike in all
derived classes. In that regard, it is like a clone() function. There is a
recent thread about how to fold this code into a policy or at least make
sure that each derived class implements the method.


BTW: I don't know very much about object oriented programming, but from what
I hear, the need for a function like superclass() or subclass() is a design
smell. What is the underlying problem that you have to solve?


Best

Kai-Uwe Bux

 

[bart van deenen]

Hi

I have a pile of objects all derived from one baseclass, and I want to
have a generic function that an object can use to see if another
object is its superclass.

[snip: something using __typeof__]

I now know more about the function of the __typeof__ operator, so I
understand why it doesn't work.

First, __typeof__ does not exists in standard C++. There is a typeof
operator, though.

But how would you do this? Is there an elegant way?

Well, for starters, there is the use of virtual functions:

class Base {
protected:

  virtual
  bool do_check ( Base * ptr ) {
    return ( true );
  }

public:

  bool subclass ( Base * ptr ) {
    return do_check( ptr );
  }

  bool superclass ( Base * ptr ) {
    return ptr->do_check( this );
  }

};

class D1 : public Base {

  bool do_check ( Base * ptr ) {
    return ( dynamic_cast< D1* >( ptr ) );
  }

};

class D2 : public D1 {

  bool do_check ( Base * ptr ) {
    return ( dynamic_cast< D2* >( ptr ) );
  }

};

class D3 : public Base {

  bool do_check ( Base * ptr ) {
    return ( dynamic_cast< D3* >( ptr ) );
  }

};
....

The next observation is that do_check() looks more or less alike in all
derived classes. In that regard, it is like a clone() function. There is a
recent thread about how to fold this code into a policy or at least make
sure that each derived class implements the method.

BTW: I don't know very much about object oriented programming, but from what
I hear, the need for a function like superclass() or subclass() is a design
smell. What is the underlying problem that you have to solve?

Thanks for your answer

My problem: I have a tree of displayable objects of different types.
These object types have an class inheritance tree, and also have the
capability to inherit attributes from their ancestor objects (in the
tree, not in the class hierarchy). A bit like html and css.

Now one of my nested objects might inherit an attribute (say 'color')
and it will have to loop through its ancestors to find one that can
provide it with the attribute 'color', but not all of its tree
ancestors might be part of its own class inheritance path, so it won't
even ask those if they have the attribute value available.

So my reasoning is that by asking another tree object if its part of
my class inheritance, is that if it says no, i don't have to ask it
for the 'color' attribute.

Hope this makes sense.

 

[Pascal J. Bourguignon]

Hi

I have a pile of objects all derived from one baseclass, and I want to
have a generic function that an object can use to see if another
object is its superclass.

[snip: something using __typeof__]

I now know more about the function of the __typeof__ operator, so I
understand why it doesn't work.

First, __typeof__ does not exists in standard C++. There is a typeof
operator, though.

But how would you do this? Is there an elegant way?

Well, for starters, there is the use of virtual functions:
[...]
BTW: I don't know very much about object oriented programming, but from what
I hear, the need for a function like superclass() or subclass() is a design
smell.

Not necessarily (in more dynamic programming languages these methods
have their use). But indeed, when programming in C++, unless you want
a dynamic programming style, it will be better to stick to virtual
methods.

Thanks for your answer

My problem: I have a tree of displayable objects of different types.
These object types have an class inheritance tree, and also have the
capability to inherit attributes from their ancestor objects (in the
tree, not in the class hierarchy). A bit like html and css.

Now one of my nested objects might inherit an attribute (say 'color')
and it will have to loop through its ancestors to find one that can
provide it with the attribute 'color', but not all of its tree
ancestors might be part of its own class inheritance path, so it won't
even ask those if they have the attribute value available.

So my reasoning is that by asking another tree object if its part of
my class inheritance, is that if it says no, i don't have to ask it
for the 'color' attribute.

Hope this makes sense.

Do use virtual methods! All the objects that can belong to this tree
shall be of a subclass of some abstract class that will publish the
virtual methods needed.

The default implementation of a method such as color() could be to
return the parent's color, and then you don't have to write the loop
explicitely.

class TreeItem
{
protected:
TreeItem* parent;
public:
static Color* defaultColor;

TreeItem(TreeItem* anItem)arent(anItem){}

virtual Color* color(){ return((parent==0)?defaultColorarent->color()); }
// ...
};


class DisplayableObjectublic TreeItem
{
public:
Color* color(){return(0);}
};

class Boxublic DisplayableObject
{
// ...
virtual Color* color(){
Color* superColor=SUPERCLASS::color();
return((superColor==0)?parent->color():superColor);
}

};

 

[bart van deenen]

bart van deenen wrote:
Hi
I have a pile of objects all derived from one baseclass, and I want to
have a generic function that an object can use to see if another
object is its superclass.
[snip: something using __typeof__]
I now know more about the function of the __typeof__ operator, so I
understand why it doesn't work.
First, __typeof__ does not exists in standard C++. There is a typeof
operator, though.
But how would you do this? Is there an elegant way?
Well, for starters, there is the use of virtual functions:
[...]
BTW: I don't know very much about object oriented programming, but from what
I hear, the need for a function like superclass() or subclass() is a design
smell.

Not necessarily (in more dynamic programming languages these methods
have their use).  But indeed, when programming in C++, unless you want
a dynamic programming style, it will be better to  stick to virtual
methods.

Thanks for your answer

My problem: I have a tree of displayable objects of different types.
These object types have an class inheritance tree, and also have the
capability to inherit attributes from their ancestor objects (in the
tree, not in the class hierarchy). A bit like html and css.

Now one of my nested objects might inherit an attribute (say 'color')
and it will have to loop through its ancestors to find one that can
provide it with the attribute 'color', but not all of its tree
ancestors might be part of its own class inheritance path, so it won't
even ask those if they have the attribute value available.

So my reasoning is that by asking another tree object if its part of
my class inheritance, is that if it says no, i don't have to ask it
for the 'color' attribute.

Hope this makes sense.

Do use virtual methods!  All the objects that can belong to this tree
shall be of a subclass of some abstract class that will publish the
virtual methods needed.  

The default implementation of a method such as color() could be to
return the parent's color, and then you don't have to write the loop
explicitely.  

class TreeItem
{
protected:
    TreeItem* parent;
public:
    static Color* defaultColor;

    TreeItem(TreeItem* anItem)arent(anItem){}

    virtual Color* color(){ return((parent==0)?defaultColorarent->color()); }
    // ...

};

class DisplayableObjectublic TreeItem
{
public:
    Color* color(){return(0);}

};

class Boxublic DisplayableObject
{
    // ...
    virtual Color* color(){
        Color* superColor=SUPERCLASS::color();
        return((superColor==0)?parent->color():superColor);
   }

};

I think I like your approach, it will just mean a huge number of
setters and getters all of them with that default behavior.

What I really don't like about this approach is that for instance a
property 'linewidth' which probably does not exist in half of the
treeitem class types now has to have an accessor function in the
toplevel superclass. That's an ugly contamination of the top
superclass, but it's probably unavoidable. :-(

Thanks for thinking along.

Bart

P.S. I've been programming a lot in Python recently, and that really
is much nicer for this kind of thing

 

[bart van deenen]

bart van deenen wrote:
Hi
I have a pile of objects all derived from one baseclass, and I want to
have a generic function that an object can use to see if another
object is its superclass.
[snip: something using __typeof__]
I now know more about the function of the __typeof__ operator, so I
understand why it doesn't work.
First, __typeof__ does not exists in standard C++. There is a typeof
operator, though.
But how would you do this? Is there an elegant way?
Well, for starters, there is the use of virtual functions:
[...]
BTW: I don't know very much about object oriented programming, but from what
I hear, the need for a function like superclass() or subclass() is a design
smell.

Not necessarily (in more dynamic programming languages these methods
have their use).  But indeed, when programming in C++, unless you want
a dynamic programming style, it will be better to  stick to virtual
methods.

Thanks for your answer

My problem: I have a tree of displayable objects of different types.
These object types have an class inheritance tree, and also have the
capability to inherit attributes from their ancestor objects (in the
tree, not in the class hierarchy). A bit like html and css.

Now one of my nested objects might inherit an attribute (say 'color')
and it will have to loop through its ancestors to find one that can
provide it with the attribute 'color', but not all of its tree
ancestors might be part of its own class inheritance path, so it won't
even ask those if they have the attribute value available.

So my reasoning is that by asking another tree object if its part of
my class inheritance, is that if it says no, i don't have to ask it
for the 'color' attribute.

Hope this makes sense.

Do use virtual methods!  All the objects that can belong to this tree
shall be of a subclass of some abstract class that will publish the
virtual methods needed.  

The default implementation of a method such as color() could be to
return the parent's color, and then you don't have to write the loop
explicitely.  

class TreeItem
{
protected:
    TreeItem* parent;
public:
    static Color* defaultColor;

    TreeItem(TreeItem* anItem)arent(anItem){}

    virtual Color* color(){ return((parent==0)?defaultColorarent->color()); }
    // ...

};

class DisplayableObjectublic TreeItem
{
public:
    Color* color(){return(0);}

};

class Boxublic DisplayableObject
{
    // ...
    virtual Color* color(){
        Color* superColor=SUPERCLASS::color();
        return((superColor==0)?parent->color():superColor);
   }

};

--

Hi Pascal

one more thing, what I dislike about this approach is that a now have
a global namespace for attribute names via the getters and setters in
the top superclass. In my approach, I can have subclasses of different
types use the same named attribute 'color' for instance, without them
biting each other. I wonder if that's even a disadvantage though.

The problem is that the whole tree structure is completely dynamic,
driven from google protocol buffer encoded data, that already has
attribute names inside it, and I'm pretty sure I'll get attribute
collisions there.

I'll have to play with it a bit.

Thanks

Bart