char * buffer = malloc( ... );
read( source, buffer );
int type = buffer[ 0 ];

void *o;
switch( type ) {
    case RECT:    o = makeRectFromString( buffer ); break;
    case ELLIPSE: o = makeEllipseFromString( buffer ); break;
    ...
}

##</code><code>##

class Rect;
bool intersect( Rect* );
bool intersect( Ellipse* );
bool intersect( Polygon* );
...