How can I remove dynamic_cast and if statements from this code snippet?

[Chris Stankevitz]

Hello,

I would like to remove the "dynamic_cast" and "if" statements from the
code below. I believe some people would describe this as making the
code "polymorphic". Can you recommend a way to do this without
modifying the original classes in "Library A"?

My intention is to create an "XML Writer" class(es) for shapes without
putting "XML code" in the base classes. I plan to use a similar
pattern for drawing and other tasks my application has to perform on
Shape objects.

Thank you,

Chris

// Library A
struct Shape { virtual ~Shape() {} };
struct Circle : public Shape { float radius; };
struct Square : public Shape { float edge; };

// Library B
#include <iostream>

class XmlWriter
{
static void write(Shape* shape)
{
if (Circle* circle = dynamic_cast<Circle*>(shape))
{
std::cout << "<Circle Radius=" << circle->radius << "/>";
}
else if (Square* square = dynamic_cast<Square*>(shape))
{
std::cout << "<Square Edge=" << square->edge << "/>";
}
}
};

 

[Ian Collins]

Hello,

I would like to remove the "dynamic_cast" and "if" statements from the
code below. I believe some people would describe this as making the
code "polymorphic". Can you recommend a way to do this without
modifying the original classes in "Library A"?

My intention is to create an "XML Writer" class(es) for shapes without
putting "XML code" in the base classes. I plan to use a similar
pattern for drawing and other tasks my application has to perform on
Shape objects.

Thank you,

Chris

// Library A
struct Shape { virtual ~Shape() {} };
struct Circle : public Shape { float radius; };
struct Square : public Shape { float edge; };

// Library B
#include<iostream>

class XmlWriter
{
static void write(Shape* shape)
{
if (Circle* circle = dynamic_cast<Circle*>(shape))
{
std::cout<< "<Circle Radius="<< circle->radius<< "/>";
}
else if (Square* square = dynamic_cast<Square*>(shape))
{
std::cout<< "<Square Edge="<< square->edge<< "/>";
}
}
};

Something like:

struct Shape {
virtual void write() = 0;
};
struct Circle : public Shape {
float radius;
void write()
{
std::cout<< "<Circle Radius="<< circle->radius<< "/>";
}
};

struct Square : public Shape {
float edge;
void write()
{
std::cout<< "<Square Edge="<< square->edge<< "/>";
}
};

class XmlWriter
{
static void write(Shape* shape)
{
shape->write();
}
};

 

[Chris Stankevitz]

Something like:

struct Shape {
   virtual void write() = 0;};

Ian,

I prefer/require a solution in which the shapes themselves are not
writing out XML -- I want the XML related code restricted to the
"XmlWriter" class in "Library B". This appears to be an overly
draconian requirement in this simple example, but should make more
sense if you consider adding something like a "drawing" or "network"
class that would introduce dependencies on drawing or socket code.

I want drawing/xml/socket code relegated to "Library B" without
compile or link dependencies in "Library A".

Thank you,

Chris

 

[red floyd]

Ian,

I prefer/require a solution in which the shapes themselves are not
writing out XML -- I want the XML related code restricted to the
"XmlWriter" class in "Library B". This appears to be an overly
draconian requirement in this simple example, but should make more
sense if you consider adding something like a "drawing" or "network"
class that would introduce dependencies on drawing or socket code.

I want drawing/xml/socket code relegated to "Library B" without
compile or link dependencies in "Library A".

You are somewhat correct here. Have the "write" function return
a string which the XML code can display. By divorcing the I/O from
the representation return, you're more general anyways.

BTW, what book are you reading? This fragment/problem is actually
one of the canonical OOP examples.

class Shape {
public:
//...
virtual std::string as_string() = 0;
}

class Square {
//...
std::string as_string() { return "Edge..." };
}

class

 

[Chris Stankevitz]

You are somewhat correct here.  Have the "write" function return
a string which the XML code can display.  By divorcing the I/O from
the representation return, you're more general anyways.

Red,

Thank you for your reply. I believe you have attempted to solve my
problem by modifying the original shapes to return their "XML
components" as strings.

Unfortunately this is not what I am interested in because it does not
scale to what I really want to do. Instead of writing a long-winded
response that might not come across correctly, allow me to change my
original question to use drawing instead of string writing:

===

Is it possible in C++ to modify "Library B" below to eliminate the
dynamic_cast and switch statements, while at the same time not doing
any of the following:
- Do not use DeviceContext in "Library A"
- Do not put drawing code in "Library A"
- Do not put the concept of drawing into "Library A" including adding
a class Shape::GetPixelsToDraw

My goal is to
a) not put any reference to drawing into Library A
b) not use dynamic_cast or switch/if blocks in Library B

The answer might be something like "use factories" or "use template"
or "this is not possible in c++".

Thank you,

Chris


// Library A
struct Shape { virtual ~Shape() {} };
struct Circle : public Shape { float radius; };
struct Square : public Shape { float edge; };

// Library B

#include <cmath>
struct DeviceContext { void FillPixel(int PixelX, int PixelY) {}; };

class Drawer
{
static void write(Shape* shape, DeviceContext& Dc)
{
if (Circle* circle = dynamic_cast<Circle*>(shape))
{
for (float Angle = 0; Angle < 2*3.14156; Angle += 0.1)
{
Dc.FillPixel(cos(Angle) * circle->radius, sin(Angle) * circle-

radius);

}
}
else if (Square* square = dynamic_cast<Square*>(shape))
{
Dc.FillPixel(0, 0);
Dc.FillPixel(square->edge, 0);
Dc.FillPixel(square->edge, square->edge);
Dc.FillPixel(0, square->edge);
}
}
};

 

[Ian Collins]

Ian,

I prefer/require a solution in which the shapes themselves are not
writing out XML -- I want the XML related code restricted to the
"XmlWriter" class in "Library B". This appears to be an overly
draconian requirement in this simple example, but should make more
sense if you consider adding something like a "drawing" or "network"
class that would introduce dependencies on drawing or socket code.

I want drawing/xml/socket code relegated to "Library B" without
compile or link dependencies in "Library A".

Well you have to look seriously at whether a shape knows how to write
its self, or an XmlWriter knows how to write a shape. If it's the
latter, you gave to provide a means for an XmlWriter to access the
required information. This can lead to significant complexity and
coupling in your design.

It gets more complex still if you consider drawing a shape.

 

[Chris Stankevitz]

        virtual void write(const serializer& writer) const

Leigh,

Thank you for your reply. I believe you have effectively modified the
base class to return the XML components as strings. This is not what
I am attempting to do as it requires me to modify the base class to
assume it will be serialized. I'm looking for an approach in which the
base class is not modified or at least is modified only in a way that
allows a Factory or Template or some other technique to give shape
details to another library. I do not want to modify the base class to
share "serializing" details. I try to explain this a little better in
my reply to Red.

Thank you,

Chris

 

[Chris Stankevitz]

Well you have to look seriously at whether a shape knows how to write
its self, or an XmlWriter knows how to write a shape.  If it's the
latter, you have to provide a means for an XmlWriter to access the
required information.

Yes, it is the latter that I would like to do.

 This can lead to significant complexity

Yes, I am looking for complexity and am curious if it is possible to
do this with c++.

It gets more complex still if you consider drawing a shape.

Yes, I am trying to draw Shapes without Shapes knowing how to draw
themselves. Is it possible to do this in c++ without using
dynamic_cast or lots of switch/if statements?

Thank you,

Chris

 

[Chris Stankevitz]

struct DrawableCircle : Circle, Drawable

Unfortunately the object that creates my shapes cannot create
DrawableShapes as that library has no dependency on or knowledge of
the concept of drawing. But thank you for continuing to attempt to
help me.

Chris

 

[Ian Collins]

Yes, it is the latter that I would like to do.


Yes, I am looking for complexity and am curious if it is possible to
do this with c++.


Yes, I am trying to draw Shapes without Shapes knowing how to draw
themselves. Is it possible to do this in c++ without using
dynamic_cast or lots of switch/if statements?

The problem has little to do with C++. The issue of who knows how to do
what is common to any language.

The literal answer to your question is yes. You can always replace
conditionals with virtual member functions.

 

[Chris Stankevitz]

You can always replace conditionals with virtual member functions.

I assume in the drawing case you are suggesting I add a virtual
function "GetVerticies" or a virtual function "Draw". This is now
what I want for these reasons:

1. I have to modify the Shape class to be aware that it is being drawn
2. Some shapes cannot be drawn with "verticies" and need to be drawn
carefully and uniquely -- for example a doughnut that has a hole in
the middle.

Thank you,

Chris

 

[Ian Collins]

I assume in the drawing case you are suggesting I add a virtual
function "GetVerticies" or a virtual function "Draw". This is now
what I want for these reasons:

1. I have to modify the Shape class to be aware that it is being drawn
2. Some shapes cannot be drawn with "verticies" and need to be drawn
carefully and uniquely -- for example a doughnut that has a hole in
the middle.

So how would a generic drawing class be expected to know this? It would
have to changed for each new shape.

Does your window manager know how to render every type of window on your
system?

 

[Chris Stankevitz]

So how would a generic drawing class be expected to know this?

This is basically the question I am asking this group.

Below is a pseudocode answer to the question that
- leaves Shape, Circle, and Square untouched (good)
- places no drawing dependency on the shapes (good)
- leaves drawing code entirely in the Drawer classes (good)
- does not have switch or if statements (good)
* does use a dynamic cast (BAD)

Can you (or anyone else) come up with some c++ pseudocode that
- leaves Shape, Circle, and Square untouched (good)
- places no drawing dependency on the shapes (good)
- leaves drawing code entirely in the Drawer classes (good)
- does not have switch or if statements (good)
- does not use a dynamic cast (good)

Chris

=====

//----------
struct Drawer
{
virtual void Draw(Shape* shape) = 0;
};

//----------
struct CircleDrawer : public Drawer
{
void Draw(Shape* shape)
{
// dynamic_cast<Circle*>(shape)
// draw the circle
}
};

//----------
struct SquareDrawer : public Drawer
{
void Draw(Shape* shape)
{
// dynamic_cast<Square*>(shape)
// draw the square
}
};

//----------
struct DrawerCreator
{
virtual Drawer* GetNewDrawer() = 0;
virtual bool CanDraw(Shape* shape) = 0;
};

//----------
struct CircleDrawerCreator : public DrawerCreator
{
Drawer* GetNewDrawer() { return new CircleDrawer; }
bool CanDraw(Shape* shape) { return dynamic_cast<Circle*>(shape); }
};

//----------
struct SqareDrawerCreator : public DrawerCreator
{
Drawer* GetNewDrawer() { return new SquareDrawer; }
bool CanDraw(Shape* shape) { return dynamic_cast<Square*>(shape); }
};

//----------
struct DrawerManager
{
void RegisterDrawerCreator(DrawerCreator* dc)
{
drawers.push_back(dc);
}

void Draw(Shape* shape)
{
if (Drawer* drawer = GetDrawer(shape))
{
drawer->Draw(shape);
}
}

Drawer* GetDrawer(Shape* shape)
{
for (int i = 0; i < drawers.size(); ++i)
{
if (drawers->CanDraw(shape))
{
return drawers->GetNewDrawer();
}
}

return 0;
}

vector<Drawer*> drawers;
};

//----------
int main()
{
DrawerManager drawer_manager;

drawer_manager.RegisterDrawerCreator(new CircleDrawerCreator);
drawer_manager.RegisterDrawerCreator(new SquareDrawerCreator);

Shape* shape1 = new Circle;
Shape* shape2 = new Square;

drawer_manager.Draw(shape1);
drawer_manager.Draw(shape2);
}

 

[Werner]

Below is a pseudocode answer to the question that
 - leaves Shape, Circle, and Square untouched (good)
 - places no drawing dependency on the shapes (good)
 - leaves drawing code entirely in the Drawer classes (good)
 - does not have switch or if statements (good)
 * does use a dynamic cast (BAD)

Can you (or anyone else) come up with some c++ pseudocode that
 - does not use a dynamic cast (good)

I can't see why using dynamic cast is so bad. It is not as
if you have an if/else style of programming that requires
constant modification. dynamic_cast under your control that
requires no modification is good (except for being a tid
slower, perhaps).

You might want to remove the burden from your client though (see
below):

#include <vector>

struct Shape{};
struct Circle : Shape{};
struct Square : Shape{};

struct AbstractDrawer
{
virtual void Draw(Shape* shape) = 0;

};

template <class ShapeT>
struct TypedDrawer : AbstractDrawer
{
typedef ShapeT shape_type;

protected:
virtual void DoDraw( shape_type& shape ) = 0;
//{ Default implementation ??? }

private:
virtual void Draw( Shape* shape )
{
DoDraw( dynamic_cast<shape_type&>(*shape) );
}
};

struct CircleDrawer : TypedDrawer<Circle>
{
protected:
virtual void DoDraw( Circle& shape )
{
//Implementation
}
};

struct SquareDrawer : TypedDrawer<Square>
{
protected:
virtual void DoDraw( Square& shape )
{
//Implementation
}
};

//----------
struct AbstractDrawerCreator
{
virtual AbstractDrawer* GetNewDrawer() = 0;
virtual bool CanDraw(Shape* shape) = 0;
};

template <class DrawerT>
struct TypedDrawerCreator : AbstractDrawerCreator
{
private:
typedef DrawerT drawer_type;
typedef typename DrawerT::shape_type shape_type;

virtual drawer_type* GetNewDrawer() const
{
return new drawer_type;
}
virtual bool CanDraw( Shape* shape ) const
{
return dynamic_cast<shape_type*>(shape);
}
};

typedef TypedDrawerCreator<CircleDrawer> CircleDrawerCreator;
typedef TypedDrawerCreator<SquareDrawer> SquareDrawerCreator;
//etc...


//[Werner Erasmus: Left this in as I've removed compiler
// errors]
struct DrawerManager
{
void RegisterDrawerCreator(AbstractDrawerCreator* dc)
{
drawers.push_back(dc);
}

void Draw(Shape* shape)
{
if (AbstractDrawer* drawer = GetDrawer(shape))
{
drawer->Draw(shape);
}
}

AbstractDrawer* GetDrawer(Shape* shape)
{
for (int i = 0; i < drawers.size(); ++i)
{
if (drawers->CanDraw(shape))
{
return drawers->GetNewDrawer();
}
}

return 0;
}

std::vector<AbstractDrawerCreator*> drawers;
};

 

[Francesco]

Hello,

I would like to remove the "dynamic_cast" and "if" statements from the
code below.  I believe some people would describe this as making the
code "polymorphic".  Can you recommend a way to do this without
modifying the original classes in "Library A"?

My intention is to create an "XML Writer" class(es) for shapes without
putting "XML code" in the base classes.  I plan to use a similar
pattern for drawing and other tasks my application has to perform on
Shape objects.

Thank you,

Chris

// Library A
struct Shape { virtual ~Shape() {} };
struct Circle : public Shape { float radius; };
struct Square : public Shape { float edge; };

// Library B
#include <iostream>

class XmlWriter
{
  static void write(Shape* shape)
  {
    if (Circle* circle = dynamic_cast<Circle*>(shape))
    {
      std::cout << "<Circle Radius=" << circle->radius << "/>";
    }
    else if (Square* square = dynamic_cast<Square*>(shape))
    {
      std::cout << "<Square Edge=" << square->edge << "/>";
    }
  }







};

Hi,

IMHO you can try one of the following approaches:
- map the type_info of the classes to some function that does the
actual work, any map will give you on the average better than linear
behavior (cascade of ifs). This applies only if you have lots of
classes, if you have only a handful it will be worse (probably).
- create a mirror hierarchy of classes (deriving or composing). Add
the virtual write method on those classes and just use those instead
of the original ones...

Obviously if you could modify the original classes, that would be the
way to go...
Bye,
Francesco

// CODE


//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Library A
struct Shape { virtual ~Shape() {} };
struct Circle : public Shape { float radius; };
struct Square : public Shape { float edge; };

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Library B
#include <iostream>
#include <cassert>

#define USE_UNORDERED_MAP

#ifdef USE_UNORDERED_MAP
#include <tr1/unordered_map>
#else
#include <map>
#endif

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Type unsafe functions!!

void WriteCircle( Shape * shape )
{
Circle * circle = static_cast< Circle * >( shape );
std::cout << "<Circle Radius=" << circle->radius << "/>";
}

void WriteSquare( Shape * shape )
{
Square * square = static_cast< Square * >( shape );
std::cout << "<Square Edge=" << square->edge << "/>";
}

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// TypeInfo class for map key

class CTypeInfo
{
public:
CTypeInfo( std::type_info const & inTypeInfo )
: mTypeInfoPtr( &inTypeInfo )
#ifdef USE_UNORDERED_MAP
, mHash( std::tr1::hash< std::string >()
( inTypeInfo.name() ) )
#endif
{ }

bool operator<( CTypeInfo const & inTypeInfo ) const
{ return mTypeInfoPtr->before( *inTypeInfo.mTypeInfoPtr ); }

bool operator==( CTypeInfo const & inTypeInfo ) const
{ return *mTypeInfoPtr == *inTypeInfo.mTypeInfoPtr; }

#ifdef USE_UNORDERED_MAP
size_t GetHash( void ) const
{ return mHash; }
#endif

private:
std::type_info const * mTypeInfoPtr;
#ifdef USE_UNORDERED_MAP
size_t mHash;
#endif

};

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Map stuff

#ifdef USE_UNORDERED_MAP
struct CTypeInfoHash
{
size_t operator()( CTypeInfo const & inTypeInfo ) const
{ return inTypeInfo.GetHash(); }
};
#endif

typedef void (*tFuncPtr)( Shape * );
#ifdef USE_UNORDERED_MAP
typedef std::tr1::unordered_map< CTypeInfo, tFuncPtr, CTypeInfoHash >
CMap;
#else
typedef std::map< CTypeInfo, tFuncPtr > CMap;
#endif


//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// XmlWriter

class XmlWriter
{
private:
// this is all thread unsafe!!!
static CMap & GetMap()
{
static CMap sMap;
return sMap;
}

static void Init()
{
static bool sInit = false;
if( !sInit )
{
CMap & map = GetMap();
map.insert( CMap::value_type( typeid( Circle ),
&WriteCircle ) );
map.insert( CMap::value_type( typeid( Square ),
&WriteSquare ) );
sInit = true;
}
}
public:
void write(Shape* shape)
{
Init();
CMap & map = GetMap();
CMap::const_iterator iter = map.find( typeid( *shape ) );
assert( iter != map.end() );
iter->second( shape );
}
};

//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// augmenting classes
// WARNING: quick example, many things to look after...

struct CShapePlus
{
virtual ~CShapePlus() {}
virtual void Write() const = 0;
};

// composition

struct CCirclePlus : public CShapePlus
{
Circle mCircle;
virtual void Write() const
{ std::cout << "<Circle Radius=" << mCircle.radius << "/>"; }

};

struct CSquarePlus : public CShapePlus
{
Square mSquare;
virtual void Write() const
{ std::cout << "<Square Edge=" << mSquare.edge << "/>"; }

};

// inheritance

struct CCirclePlus2 : public Circle, public CShapePlus
{
virtual void Write() const
{ std::cout << "<Circle Radius=" << this->radius << "/>"; }
};

struct CSquarePlus2 : public Square, public CShapePlus
{
virtual void Write() const
{ std::cout << "<Square Edge=" << this->edge << "/>"; }
};



//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// main

int main()
{
Shape * shape1 = new Circle;
Shape * shape2 = new Square;

XmlWriter writer;

writer.write( shape1 ); std::cout << '\n';
writer.write( shape2 ); std::cout << '\n';


CShapePlus * shape3 = new CCirclePlus;
CShapePlus * shape4 = new CSquarePlus;
CShapePlus * shape5 = new CCirclePlus2;
CShapePlus * shape6 = new CSquarePlus2;

shape3->Write(); std::cout << '\n';
shape4->Write(); std::cout << '\n';
shape5->Write(); std::cout << '\n';
shape6->Write(); std::cout << '\n';
}

// END CODE

 

[Goran]

So how would a generic drawing class be expected to know this?

This is basically the question I am asking this group.

Below is a pseudocode answer to the question that
 - leaves Shape, Circle, and Square untouched (good)
 - places no drawing dependency on the shapes (good)
 - leaves drawing code entirely in the Drawer classes (good)
 - does not have switch or if statements (good)
 * does use a dynamic cast (BAD)

Can you (or anyone else) come up with some c++ pseudocode that
 - leaves Shape, Circle, and Square untouched (good)
 - places no drawing dependency on the shapes (good)
 - leaves drawing code entirely in the Drawer classes (good)
 - does not have switch or if statements (good)
 - does not use a dynamic cast (good)

Chris

=====

//----------
struct Drawer
{
  virtual void Draw(Shape* shape) = 0;

};

//----------
struct CircleDrawer : public Drawer
{
  void Draw(Shape* shape)
  {
    // dynamic_cast<Circle*>(shape)
    // draw the circle
  }

};

//----------
struct SquareDrawer : public Drawer
{
  void Draw(Shape* shape)
  {
    // dynamic_cast<Square*>(shape)
    // draw the square
  }

};

//----------
struct DrawerCreator
{
  virtual Drawer* GetNewDrawer() = 0;
  virtual bool CanDraw(Shape* shape) = 0;

};

//----------
struct CircleDrawerCreator : public DrawerCreator
{
  Drawer* GetNewDrawer() { return new CircleDrawer; }
  bool CanDraw(Shape* shape) { return dynamic_cast<Circle*>(shape); }

};

//----------
struct SqareDrawerCreator : public DrawerCreator
{
  Drawer* GetNewDrawer() { return new SquareDrawer; }
  bool CanDraw(Shape* shape) { return dynamic_cast<Square*>(shape); }

};

//----------
struct DrawerManager
{
  void RegisterDrawerCreator(DrawerCreator* dc)
  {
    drawers.push_back(dc);
  }

  void Draw(Shape* shape)
  {
    if (Drawer* drawer = GetDrawer(shape))
    {
      drawer->Draw(shape);
    }
  }

  Drawer* GetDrawer(Shape* shape)
  {
    for (int i = 0; i < drawers.size(); ++i)
    {
      if (drawers->CanDraw(shape))
      {
        return drawers->GetNewDrawer();
      }
    }

    return 0;
  }

  vector<Drawer*> drawers;

};

//----------
int main()
{
  DrawerManager drawer_manager;

  drawer_manager.RegisterDrawerCreator(new CircleDrawerCreator);
  drawer_manager.RegisterDrawerCreator(new SquareDrawerCreator);

  Shape* shape1 = new Circle;
  Shape* shape2 = new Square;

  drawer_manager.Draw(shape1);
  drawer_manager.Draw(shape2);







}

What you seem to be showing here is the adapter pattern. The way you
went about it seems backwards, though. When you create your adapter
(Drawer), you should strive to do it using the correct adaptee type.
But what you did is to throw said type back to base (by passing Shape*
to DrawerManager) and reached for dynamic_cast.

So how about this:

int main()
{
Canvas c;

Circle shape1;
Square shape2;

Draw(c, CircleDrawer(shape1));
Draw(c, SquareDrawer(shape2));
}

class Canvas { /*Driving primitives here*/ };

class Drawer
{
public: virtual void Draw(Canvas& c) const = 0;
};

void Draw(Canvas& c, Drawer& d)
{
d.Draw(c);
}

// Trivial intermediary: drawer for Shapes.
class ShapeDrawer
{
const Shape& s_;
public:
ShapeDrawer(Shape& s) : s_(s) {}
};

// Implementation starts here

class CircleDrawer : public ShapeDrawer
{
public:
CircleDrawer(const Circle& s) : ShapeDrawer(c) {}
virtual void Draw(Canvas& c) const {knock yourself out}
};

class SquareDrawer {you get the picture};

IOW... __Don't__ use generic Shapes in lib B logic. __Adapt__ them as
soon as you get them (because then you should know their types, and
that's the only way to avoid casting) and use adapters. Added bonus:
adapters will let you mix your own stuff with lib A stuff.

Goran.

P.S. You didn't want dynamic_cast, but rather static_cast, there. ;-)

 

[Goran]

I can't see why using dynamic cast is so bad. It is not as
if you have an if/else style of programming that requires
constant modification. dynamic_cast under your control that
requires no modification is good (except for being a tid
slower, perhaps).

Well, conceptually, there's no difference between if/else and
dynamic_cast (and indeed OP's first post is dynamic_cast with if/
else). dynamic_cast merely moves decision to the type itself (whereas
if/else decides on the data). IOW, with dynamic_cast, type is used as
data. And indeed, if new type introduced, conditional logic must be
changed with itand therefore open/closed principle is compromised.

Goran.

 

[Chris Stankevitz]

Werner,

Thank you for your reply.

I can't see why using dynamic cast is so bad.

Perhaps it is not bad. One way or another I am going to complete this
task and I know I can complete it using dynamic_casts so I might end
up using them. I suppose my question to the group is "is it possible
to do something like this without using a dynamic_cast." The answer
to that question might be "no" -- which I'm okay with.

You might want to remove the burden from your client though

Thank you your method is certainly cleaner than mine. I need to look
at it a bit more closely but I believe we are both doing the same
thing.

Chris

 

[Chris Stankevitz]

Francesco,

Thank you for your reply.

- map the type_info of the classes to some function

This is the sort of thing I came to the group to hear. I've never
heard of type_info before, so I am going to go learn about it. Just
from the name my suspicion is that type_info is doing the same thing
as dynamic_cast under the covers, but I'm going to find out.

Thank you for providing a novel solution to my problem that satisfies
my conditions!

Chris

 

[Chris Stankevitz]

Goran,

What you seem to be showing here is the adapter pattern. The way you
went about it seems backwards, though.

Thank you this is exactly the kind of information I was looking for.
I'm going to go read up on Adapters. I've never heard of 'adapter
pattern'.

But what you did is to throw said type back to base (by passing Shape*
to DrawerManager) and reached for dynamic_cast.

I noticed this after I posted and some others pointed that out here on
the list. I understand the concept and the idea behind sending the
correct type to the drawer classes.

Thank you,

Chris