[toc]

参考

block - 捕获 __block对象类型

OC代码

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        __block NSObject *obj = [[NSObject alloc] init];
        NSLog(@"1_obj = %@ - %p", obj, &obj);

        void (^block)(void) = ^{
            obj = [[NSObject alloc] init];
            NSLog(@"2_obj = %@ - %p", obj, &obj);
        };
      
        NSLog(@"3_obj = %@ - %p", obj, &obj);

        block();
        NSLog(@"4_obj = %@ - %p", obj, &obj);
    }
    return 0;
}

MRC 输出: (对象地址 - 指针地址)
1_obj = <NSObject: 0x10283a9c0> - 0x7ffeefbff408 //  __block变量指针始终在栈上
3_obj = <NSObject: 0x10283a9c0> - 0x7ffeefbff408 
2_obj = <NSObject: 0x102838eb0> - 0x7ffeefbff408
4_obj = <NSObject: 0x102838eb0> - 0x7ffeefbff408

ARC 输出:
1_obj = <NSObject: 0x103a06f40> - 0x7ffeefbff408 // 变量定义时，__block变量指针在栈上
3_obj = <NSObject: 0x103a06f40> - 0x103a055a8 // 在block定义后, __block变量指针变为堆地址
2_obj = <NSObject: 0x103a05500> - 0x103a055a8
4_obj = <NSObject: 0x103a05500> - 0x103a055a8

C++代码

MRC/ARC 编译后代码一致:

int main(int argc, const char * argv[]) {
    { __AtAutoreleasePool __autoreleasepool; 
            // __block对象类型变量  __Block_byref_obj_0 obj
        __attribute__((__blocks__(byref))) __Block_byref_obj_0 obj = {(void*)0,(__Block_byref_obj_0 *)&obj, 33554432, sizeof(__Block_byref_obj_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("alloc")), sel_registerName("init"))};
     
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_p5_mp3284bs2xb073r91w__n99r0000gn_T_main_2b7f28_mi_0, (obj.__forwarding->obj), &(obj.__forwarding->obj));
                
            // 定义block, 捕获的是结构体 val 的地址, 作为第3个入参
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_obj_0 *)&obj, 570425344));
     
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_p5_mp3284bs2xb073r91w__n99r0000gn_T_main_2b7f28_mi_2, (obj.__forwarding->obj), &(obj.__forwarding->obj));
     
        // 调用block
        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
     
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_p5_mp3284bs2xb073r91w__n99r0000gn_T_main_2b7f28_mi_3, (obj.__forwarding->obj), &(obj.__forwarding->obj));
    }
    return 0;
}

// __block NSObjcet *obj = [[NSObjcet alloc] init]; 
// 对象类型的 __block 变量被编译器包装后的结构体
struct __Block_byref_obj_0 {
    void *__isa; // 
    __Block_byref_obj_0 *__forwarding;
    int __flags;
    int __size;
    void (*__Block_byref_id_object_copy)(void*, void*);  // 对象类型比基本类型多这2个成员 ★
    void (*__Block_byref_id_object_dispose)(void*); // ★
    NSObject *__strong obj; // 结构体内部保存的原始变量
};


struct __main_block_impl_0 {
    struct __block_impl impl;
    struct __main_block_desc_0* Desc;
    __Block_byref_obj_0 *obj; // by ref
    __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_obj_0 *_obj, int flags=0) : obj(_obj->__forwarding) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};

// block要执行的代码
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
    __Block_byref_obj_0 *obj = __cself->obj; // bound by ref
    (obj->__forwarding->obj) = ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("alloc")), sel_registerName("init"));
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_p5_mp3284bs2xb073r91w__n99r0000gn_T_main_2b7f28_mi_1, (obj->__forwarding->obj), &(obj->__forwarding->obj));
}


static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {
    _Block_object_assign((void*)&dst->obj, (void*)src->obj, 8/*BLOCK_FIELD_IS_BYREF*/);
}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {
    _Block_object_dispose((void*)src->obj, 8/*BLOCK_FIELD_IS_BYREF*/);
}