Objective-C属性表达式的地址 [英] Objective-C Address of property expression

问题描述

我需要财产的访问地址，但是有问题.示例代码是

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是请求的属性表达式的地址

&rect.x is Address of property expression requested

推荐答案

如注释所示，您不能使用属性的地址.属性实际上只是一个承诺，即所讨论的对象为某些值提供访问器.该值本身可能存在也可能不存在于实例变量中.例如，名为fullName的属性的吸气剂可能会通过将firstName和lastName属性的值进行串联来即时生成所需的值.

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.

由于您需要将SDL_Rect的地址传递到SDL_BlitSurface()，因此您可以先将必要的属性复制到局部变量中，然后再传递该变量的地址:

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);

如果您需要保留对SDL_BlitSurface()的调用后留在wall中的值，请在调用之后再次将其复制回:

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;

这篇关于Objective-C属性表达式的地址的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！