Objective-C Address of property expression - objective-c

Question

I need access address of property but have problem. example code is
#interface Rectangle : NSObject
{
SDL_Rect wall;
SDL_Rect ground;
}
#property SDL_Rect wall;
#property SDL_Rect ground;
#end
#implementation Rectangle
#synthesize x;
#synthesize y;
#end
#interface Graphics : NSObject
{
int w;
int h;
}
-(void) drawSurface
#end
#implementation Graphics
-(void) drawSurface
{
Rectangle *rect = [[Rectangle alloc] init];
SDL_BlitSurface(camera, NULL, background, &rect.wall);
}
#end
&rect.x is Address of property expression requested

Answer 1

As the comments suggest, you cannot take the address of a property. A property is really just a promise that the object in question provides accessors for some value. The value itself may or may not even exist in an instance variable. For example, the getter for a property called fullName might generate the required value on the fly by concatenating the values of firstName and lastName properties.
Since you need to pass the address of a SDL_Rect into SDL_BlitSurface(), you could first copy the necessary property into a local variable, and then pass the address of that variable:
Rectangle *rect = [[Rectangle alloc] init];
SDL_Rect wall = rect.wall;
SDL_BlitSurface(camera, NULL, background, &wall);
If you need to preserve the value left in wall after the call to SDL_BlitSurface(), copy it back again after the call:
rect.wall = wall;

Answer 2

I had a similar situation with subclasses needing to access a CGAffineTransform defined in the parent class. The answer came from #orpheist's answer to this question: Get the address of an Objective-c property (which is a C struct). It does involve adding a method to your Rectangle class.
#interface Rectangle : NSObject
{
NSRect wall;
NSRect ground;
}
#property NSRect wall;
#property NSRect ground;
#end
#implementation Rectangle
#synthesize wall = _wall; //x;
#synthesize ground = _ground; //y;
- (const NSRect *) addressOfWall {
return &_wall;
}
- (const NSRect *) addressOfGround {
return &_ground;
}
+(instancetype)standardRectangle
{
Rectangle *newInstance = [[self alloc] init];
newInstance.wall = NSMakeRect(0,0, 300, 100);
newInstance.ground = NSMakeRect(0 ,0, 300, 450);
return newInstance;
}
#end
Now you can use, for instance, addressOfWall thus:
- (void)testWall
{
Rectangle *rect = [Rectangle standardRectangle];
XCTAssertEqual(100, [rect addressOfWall]->size.height);
}

Answer 3

Address of property expression requested that means:
#preperty (nonatomic,copy) NSString *name;
if you want to get the address of self.name. You cannot write the code like this:
NSLog (#"%p",&(self.name));
Because in fact，self.name is getter method, like this:
- (NSString *)name {
return _name;
}
so you cannot get address of method.