在目标C中转发声明结构 [英] Forward declare structs in Objective C

问题描述

我正在编写一个iOS应用程序，其中有一个模型类，该模型类将使用提供给它的XMLElement进行初始化.

I'm writing an iOS app in which I have a model class that is going to initialize itself with an XMLElement I give to it.

我正在将TBXML用于XML部分.

I'm using TBXML for the XML part.

模型类的标题如下:

@interface CatalogItem : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSManagedObject *group;

-(id)initWithXMLElement:(TBXMLElement*)element;

@end

现在，我不想包含在其中定义TBXMLElement的标头，而是想在类定义之前用struct TBXMLElement进行声明.但是，我收到一个预期类型"错误，告诉我我的声明不起作用.这不是我要的吗?

Now instead of including the header in which TBXMLElement is defined, I'd like to forward declare it with: struct TBXMLElement before the class definition. I'm however getting an "Expected Type" error wich tells me my declaration isn't working. Is this not how I would got about this?

据我了解，将标头文件包含在标头文件中是不好的做法.编译器不需要知道TBXMLElement的内部工作原理，只需知道它在编译时已存在或将存在.

As I understand it, including header files in header files is bad practice. The compiler doesn't need to know the inner workings of TBXMLElement, just that it exists or will exist at compile time.

推荐答案

结构的前向声明一直使用，但仍涉及导入标头.这样做的动机是不允许开发人员直接进入该结构. IE.看CFString.它作为结构实现，但是您不能直接触摸结构内容.取而代之的是，有一个用于处理结构内容的完整API.这样，CFString的实现细节就可以更改，而不会破坏二进制兼容性.

Forward declaration of structs are used all the time, but still involves importing a header. The motivation is to not allow developers to dip into the structure directly. I.e. look at CFString. It is implemented as a struct, but you can't touch the structure contents directly. Instead, there is a full API for manipulating the struct contents. This allows CFString's implementation details to change without breaking binary compatibility.

在标头中(理想情况下，标头定义了与TBXMLElement*相关联的任何API):

In your header (ideally the header that defines whatever API is associated with TBXMLElement*):

TBXMLElement.h:

TBXMLElement.h:

typedef const struct TBLXMLElement *TBXMLElementRef;

extern TBXMLElementRef TBLXMLCreateElement();
... etc ...

然后，在包含TBLXMLElementAPI实现的实现文件中:

Then, in the implementation file containing the implementation of the TBLXMLElementAPI:

TBXMElement.c(假设它是C文件):

TBXMElement.c (assuming it is a C file):

typedef struct __TBLXMLElement {
    ... struct members here ...
} TBLXMLElement;

TBXMLElementRef TBLXMLCreateElement()
{
     return (TBXMLElementRef)malloc(sizeof(TBLXMLElement));
}
... etc ....

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