Objective-C Address of property expression

问题:

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

回答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; 

回答2:

Address of property expression requested that means

@preperty (nonatomic,copy) NSString *name;

if you want to get 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.

回答3:

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); } 

文章来源: Objective-C Address of property expression