Question

I have a base class line which has a child class arc (note that additional classes will inherit from line)… I need to store a list, or vector of line objects that may be a line or an arc. arc has an additional property that line does not. So as I’m working with my lists, or vector of line objects, how do I determine whether the object is a line or an arc?

I have prepared a small sample program and put in comments with pseudo-code a sample of what I would be trying to accomplish. Is this even possible?

#include <iostream>
#include <vector>
#include <iterator>

using namespace std;

struct point
{
    double x;
    double y;
};

class line
{
    public:

        point start;
        point end;

};

class arc: public line
{
    public:

        point center;

};


int main( int argc, char* argv[] )
{


    vector<line*> a;
    a.push_back( new line );
    a.push_back( new arc );

    for( vector<line*>::iterator i = a.begin(); i < a.end(); i++ )
    {
        (*i)->start.x = 10;
        (*i)->start.y = 11;
        (*i)->end.x = 101;
        (*i)->end.y = 102;
        //if( type of (*i) is arc ) 
        //{
        //  (i)->center.x = 111;
        //  (i)->center.y = 112;
        // }
    }

    return 0;

}

Answer 1

In your loop, try this:

arc *ar = dynamic_cast<arc *>(*i);
if ( NULL != ar ) {
 // it's an arc!
}else {
  // it's just a line
}

Sid is also right…use virtual functions for this instead. Using dynamic_cast for simple cases such as this is generally considered poor style.