Arithmetic of fast pointers and dereferencing; converting some C-like map code to Swift

Question

I have some Swift code that doesn't seem to work ...

// earlier, in Obj C... typedef struct _Room { uint8_t *map; int width; int height; } Room; 

The room is part of the kick in the game with roguelike, if you're interested. I am trying to rewrite a couple of parts in Swift. Here's a code that looks broken, and what I hope I do as comments:

 let ptr = UnsafePointer<UInt8>(room.map) // grab a pointer to the map out of the room struct let offset = (Int(room.width) * Int(point.y)) + Int(point.x) // calculate an int offset to the location I am interested in examining let locationPointer = ptr + offset // pointer advances to point to the offset I want var pointValue = ptr.memory // What I used to get with *ptr 

Something is wrong here, because simple tests show that the pointValue value is not what I know, I look at the map by setting a very simple location (1, 1) to a known value. It seems pretty obvious that Swift shouldn't do this kind of thing, but it's a conversion to learn Way of Swift when I know the syntax well.

I expect the error is indicated in the quick code, as it all worked in the objective version of C. Where is the error?

Answer 1

You assign locationPointer to point to the new location, but still use ptr in the next line, and the ptr value has not been changed. Change your last line to:

 var pointValue = locationPointer.memory 

or you can change the pointer to var and advance it:

 var ptr = UnsafePointer<UInt8>(room.map) // grab a pointer to the map out of the room struct let offset = (Int(room.width) * Int(point.y)) + Int(point.x) // calculate an int offset to the location I am interested in examining ptr = ptr + offset // pointer advances to point to the offset I want var pointValue = ptr.memory // What I used to get with *ptr