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Start your free trialJamie Wyton
3,011 Pointsthis
using the new 'this' command has got me lost a bit. I'm ok with it being used in the MapLocation constructor and that it takes the current object as the argument to be carried over as it calls on the OnMap method. But on the receiving end of OnMap the argument type is Point point! How is this compatible?
1 Answer
andren
28,558 PointsIt's compatible because MapLocation
is a class that inherits from Point
as you can see if you look at it's declaration:
class MapLocation : Point
The : Point
means that it inherits from Point
. The fact that it inherits from Point
means that a MapLocation
object can safely be treated as a Point
object since it contains all methods and properties that a Point
object would have.
Jamie Wyton
3,011 PointsJamie Wyton
3,011 Pointsso in the case of the arguments (int x, int y, Map map) - would they then be discarded?
andren
28,558 Pointsandren
28,558 PointsAny property unique to the
MapLocation
class would not be accessible while it is being classified as aPoint
object.That means
map
would indeed not be accessible,x
andy
on the other actually would be. Because those arguments are passed to thePoint
constructor when theMapLocation
object is created.public MapLocation(int x, int y, Map map) : base(x, y)
The
: base(x, y)
part of the constructor tells C# to call the constructor of the parent class (Point
in this case) with the arguments provided. Sox
andy
are passed toPoint
which means that those are present even when the object is treated as aPoint
.Whenever a class inherits from another class it always has to call the constructor of it's parent to pass it whatever arguments it needs to create a valid instance of that class.
Put as simply as possible
MapLocation
is aPoint
object that just has some properties and methods not present inPoint
, all that happens when it gets treated (cast is the technical term) as aPoint
object is that those unique properties and methods are no longer accessible. SinceMapLocation
is guaranteed to have all of the properties and methods that thePoint
class has it is completely safe to treat it as that object.