Objective-C 是否禁止使用结构体？
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Does Objective-C forbid use of structs?
提问by user2490003
I'm new to Objective C
我是目标 C 的新手
I tried using a simple structand got
我尝试使用一个简单的struct并得到
arc forbids objective-c objects in struct
Looking up ARC, it looks like this is the specification that defines Objective C syntaxt - is that correct?
查找 ARC，看起来这是定义 Objective C 语法的规范 - 对吗？
Secondly, how do I go about using structif it's not allowed?
其次，struct如果不允许，我该如何使用？
Thank you!
谢谢！
Edit: Some code as a sample
编辑：一些代码作为示例
@implementation Cities {
// The goal is to have a struct that holds information about a city,
// like when a person started and ended living there.
// I was trying to make this struct an instance variable of the Cities
// class
// XCode doesn't like the below struct definition
struct City
{
NSString *name;
int *_startYear;
int *_endYear;
};
}
回答by dandan78
arc forbids objective-c objectsin struct
arc 禁止结构中的objective-c 对象
Structs are a C construct. The compiler is telling you, in very unabiguous terms, that you can't have Objective-C objects inside a struct, not that structs are illegal.
结构是一个 C 结构。编译器非常明确地告诉您，结构中不能有 Objective-C 对象，而不是结构是非法的。
You can use regular C structs all you want.
您可以随心所欲地使用常规的 C 结构。
Your example tries to put references to an Objective-C object, NSString, into a struct, which is incompatible with ARC.
您的示例尝试将对 Objective-C 对象 , 的引用NSString放入struct与 ARC 不兼容的 。
Structs are typically used for simple data structures. Examples that you are likely to come across in Objective-C code are CGPointand CGRect.
结构通常用于简单的数据结构。您可能在 Objective-C 代码中遇到的示例是CGPoint和CGRect。
CGPointlooks something like this
CGPoint看起来像这样
struct CGPoint
{
CGFloat x;
CGFloat y;
};
A CGFloatis, I think, just a double, and the idea it to represent a point in 2D space. Structs can include pointers to other structs, C-arrays and standard C data types such as int, char, float... And Objective-C classes can contain structs, but the reverse does not work.
ACGFloat是，我认为，只是一个double，它代表二维空间中的一个点的想法。结构体可以包含指向其他结构体、C 数组和标准 C 数据类型的指针，例如int, char, float... 并且 Objective-C 类可以包含结构体，但反过来不起作用。
Structs can also get pretty complicated, but that is a very broad topic that is best researched using Google.
结构也可能变得非常复杂，但这是一个非常广泛的主题，最好使用 Google 进行研究。
回答by Kazuki Sakamoto
You can use structin Objective-C++with ARC in any case.
在任何情况下，您都可以struct在带有 ARC 的Objective-C++ 中使用。
#import <Foundation/Foundation.h>
@interface City : NSObject
struct Data {
NSString *name;
};
@property struct Data data;
@end
@implementation City
@end
int main()
{
City *city = [[City alloc] init];
city.data = (struct Data){@"San Francisco"};
NSLog(@"%@", city.data.name);
return 0;
}
If you compile it as Objective-C, you failed as you said.
如果你把它编译成Objective-C，你说的就失败了。
$ clang -x objective-c -fobjc-arc a.m -framework Foundation
a.m:5:15: error: ARC forbids Objective-C objects in struct
NSString *name;
^
1 error generated.
Because C struct doesn't have the capability of management for variable life span.
因为C struct 没有管理可变生命周期的能力。
But in C++, struct does have destructor function. So C++ struct is compatible with ARC.
但是在C++中，struct确实有析构函数。所以 C++ struct 与 ARC 兼容。
$ clang++ -x objective-c++ -fobjc-arc a.m -framework Foundation
$ ./a.out
San Francisco
回答by Soumen
If you want to use struct in Objective C ( with ARC) use "__unsafe_unretained" attribute.
如果您想在 Objective C（带有 ARC）中使用 struct，请使用“__unsafe_unretained”属性。
struct Address {
__unsafe_unretained NSString *city;
__unsafe_unretained NSString *state;
__unsafe_unretained NSString *locality;
};
