Clang -rewrite-objc

在Objective-C类成员变量深度剖析一文中，作者通过分析Clang生成的LLVM中间码得出了如下结论：

结论是对的，但要读LLVM中间码，不免令人头大。一个简单的办法是用

Clang -rewrite-objc *.m

将OC代码转换成C++代码，然后分析C++代码即可。

Let's do it

以Command Line Tool模板为例，比如

//Dummy.h
@interface Dummy : NSObject {
@public
    int myInt;
}

@end

//Dummy.m
@implementation Dummy
@end

在main.m中，有

#import <Foundation/Foundation.h>
#import "Dummy.h"

int main(int argc, const char * argv[]) {  
    Dummy *dummy = [Dummy new];
    dummy->myInt;
      
    return 0;
}

执行

Clang -rewrite-objc main.m

打开生成的main.cpp，可以看到

//Foundation.h展开后的一大堆C++定义
//...

/* @end */

#ifndef _REWRITER_typedef_Dummy
#define _REWRITER_typedef_Dummy
typedef struct objc_object Dummy;
typedef struct {} _objc_exc_Dummy;
#endif

extern "C" unsigned long OBJC_IVAR_$_Dummy$myInt;
struct Dummy_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
    int myInt;
};

/* @end */
int main(int argc, const char * argv[]) {
    Dummy *dummy = ((Dummy *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Dummy"), sel_registerName("new"));
    (*(int *)((char *)dummy + OBJC_IVAR_$_Dummy$myInt));

    return 0;
}

其中OBJC_IVAR_$_Dummy$myInt就是存储Dummy的myInt成员变量偏移值的全局变量。

如果用@property，会稍微复杂一些，比如

//Dummy.h
@interface Dummy : NSObject
@property (nonatomic, assign) int myInt;

@end

//Dummy.m
@implementation Dummy
@end

执行

Clang -rewrite-objc Dummy.m

打开生成的Dummy.cpp，可以看到

extern "C" unsigned long OBJC_IVAR_$_Dummy$_myInt;
struct Dummy_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
    int _myInt;
};

// @property (nonatomic, assign) int myInt;

/* @end */


// @implementation Dummy



static int _I_Dummy_myInt(Dummy * self, SEL _cmd) { return (*(int *)((char *)self + OBJC_IVAR_$_Dummy$_myInt)); }
static void _I_Dummy_setMyInt_(Dummy * self, SEL _cmd, int myInt) { (*(int *)((char *)self + OBJC_IVAR_$_Dummy$_myInt)) = myInt; }
// @end

对应property的setter和getter，Clang相应地生成了两个static function，两者都通过OBJC_IVAR_$_Dummy$myInt来访问myInt。