EXC_BAD_ACCESS memory issue when accessing instance variables - objective-c

Question

I'm having issues porting a Java project I worked on a while ago to Objective-C code - I get "Program received signal: "EXC_BAD_ACCESS"", on the first line in this code:
-(Point3D *) unit {
NSLog(#"%#%#", "X: ", x);
double length = [self length];
return [[Point3D alloc] initWithX:x/length andY:y/length andZ:z/length];
}
called from here:
-(id) initWithStart:(Point3D *)start andDirection:(Point3D *)dir {
if ( self = [super init] ) {
NSLog(#"%#%#", #"Direction:", [dir toString]);
printf("Trying to find unit length of direction...\n");
NSLog(#"%#", [[dir unit] toString]);
self.start = start;
self.direction = [dir unit];
}
return self;
}
Console output is:
2011-12-09 17:20:14.021 RayTracerProject[16607:407] Direction:(0,0,20)
Trying to find unit length of direction...
The toString method of the Point3D looks like this:
-(NSString *) toString {
NSNumber *xstring = [NSNumber numberWithDouble:self.x];
NSNumber *ystring = [NSNumber numberWithDouble:self.y];
NSNumber *zstring = [NSNumber numberWithDouble:self.z];
NSString * str = #"(";
str = [str stringByAppendingString:[xstring stringValue]];
str = [str stringByAppendingString:#","];
str = [str stringByAppendingString:[ystring stringValue]];
str = [str stringByAppendingString:#","];
str = [str stringByAppendingString:[zstring stringValue]];
str = [str stringByAppendingString:#")"];
return str;
}
So, from what I can see, my (Point3D *) dir is alive and well when I check what the value is using my [dir toString] call. But when I try to call [dir unit], it seems I no longer have the variables I did in the object, hence the EXC_BAD_ACCESS error.
What am I doing wrong here? I think it's something to do with the way I'm managing (or not) my memory usage, but I don't know what it is.

Answer 1

The NSLog in your method 'unit' should be:
NSLog(#"%# %g", #"X:", x);
Moving the space is optional, but just makes it clearer. The key issues were that you were missing the # before the "X: ", and you need to use %g rather than %# because from your other code x is a double rather than an NSObject. Equivalent—but simpler and thus better—would also be:
NSLog(#"X: %g", x);
Apple's documentation provides the definitive guide to string format specifiers and that and the surrounding documentation explain the use of format specifiers like %# (to reference an Objective C object) and %g (to reference a 64-bit floating-point number) when constructing an NSString.

Answer 2

It seems this line here:
NSLog(#"%#%#", "X: ", x);
should look more like this:
NSLog(#"%#%lf", #"X: ", self.x);
Note a few changes here:
From your other code, it seems you have a property named 'x'. self.x references that property. (You may or may not also have an instance variable named 'x')
Also from your other code, it seems your property 'x' is a double. So the placeholder for a double is %lf, not %#. %# is for NSObjects (and descendants)
NSString literals start with #. So #"X: " is an NSString literal. What you had is a C string.
EDIT
Well, I'm slow as usual. :) Ignore what I said and accept Duncan's answer, which was faster and now cleaner, actually.