创建一个宏以执行默认初始化 [英] Creating a macro to perform default init

问题描述

我有很多重复此简单样板的方法:

I have a lot of methods that repeat this simple boilerplate:

- (id)myObject {
    if(!_myObject) {
        self.myObject = [_myObject.class new];
    }
    return _myObject;
}

所以我想用一个简单的宏替换它:

So I want to replace this with a simple macro:

#define default_init(instance) \
    if(!instance) instance = [instance.class new]; \
    return instance;

这样我只需要打电话:

- (id)myObject {
        default_init(_myObject);
}

上面的代码当前可以编译，但是问题是宏直接设置实例变量的值.相反，我想称呼self.instance = value;

The above code currently compiles, but the issue is that the macro directly sets the instance variable's value. Instead, I'd like to call self.instance = value;

所以不是

if(!instance) instance = [instance.class new];

我想要类似的东西

if(!instance) self.instance = [instance.class new];

但是显然，当前代码不允许这样做.我该如何完成这样的事情?

But obviously the current code does not allow for this. How might I accomplish something like this?

推荐答案

使用此宏:

#define default_init(class, instance)      \
    if ( ! _##instance ) {                 \
        self.instance = [class new] ;      \
    }                                      \
    return _##instance

我能够创建此实例方法:

I was able to create this instance method:

- (NSMutableArray*) myObject {
    default_init(NSMutableArray, myObject) ;
}

我必须添加一个定义类的参数，因为_myObject仍然是nil，因此_myObject.class是nil.

I had to add a parameter defining the class, because _myObject is still nil, therefore _myObject.class is nil.

此StackOverflow问题和

This StackOverflow question and this Cprogramming page recommend wrapping your multi-line macro in do {...} while(0):

#define default_init(class, instance)          \
    do {                                       \
        if ( ! _##instance ) {                 \
            self.instance = [class new] ;      \
        }                                      \
        return _##instance ;                   \
    } while(0)

如果确实愿意，可以创建一个定义整个方法的宏:

If you really wanted to, you could make a macro that defines the entire method:

#define default_getter(class, instance)        \
    - (class*) instance {                      \
        if ( ! _##instance ) {                 \
            self.instance = [class new] ;      \
        }                                      \
        return _##instance ;                   \
    }

然后使用它:

default_getter(NSMutableArray, myObject)

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