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

From:

Ian Collins <ian-news@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 17 Nov 2011 07:52:41 +1300

Message-ID:

<9iif3pFi5oU11@mid.individual.net>

On 11/17/11 07:37 AM, Chris Stankevitz wrote:

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();
   }
};

--
Ian Collins