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When Does an Objective-C Object Held by a C++ Class Get dealloc'd?C++ 类中持有的 Objective-C 对象何时 dealloc

2020-04-17 · Performance & internals

Translated from Chinese — .

On this page

• References
• Summary

When using Objective-C++, you can store Objective-C objects in a C++ struct or class.

For example:

struct CppStruct {
    MyObject *obj;
};

So I suddenly wondered: if CppStruct is destructed, will MyObject be dealloc’d? No need to overthink it — of course it will. But how is that achieved?

Let’s write some code to verify it:

@interface MyObject : NSObject
@end
@implementation MyObject
- (instancetype)init {
    self = [super init];
    if (self) {
        NSLog(@"MyObject init");
    }
    return self;
}
- (void)dealloc {
    NSLog(@"MyObject dealloc");
}
@end

struct CppStruct {
    MyObject *obj;
    CppStruct() {
        NSLog(@"CppStruct constructor");
    }
    ~CppStruct() {
        NSLog(@"CppStruct destructor");
    }
};

CppStruct * CreateStruct() {
    CppStruct * s = new CppStruct();
    s->obj = [[MyObject alloc] init];
    return s;
}

void FreeStruct(CppStruct *s) {
    delete s;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        CppStruct *s = CreateStruct();
        FreeStruct(s);
    }
    return 0;
}

It produces the following output:

As you can see, MyObject indeed got dealloc’d.

So, let’s set a breakpoint on dealloc and see how it gets called.

In the image above there are two ~CppStruct(), one of which calls objc_storeStrong, and objc_storeStrong in turn triggers MyObject’s dealloc.

From the image above, it must be the compiler that generated the code calling objc_storeStrong.

Looking at the Disassembly, it indeed generated the objc_storeStrong call code.

Solid confirmation.

As for how LLVM generates this logic — I won’t dig into that here.

References

The debugging above used a compilable objc runtime. There are many on GitHub; you can just search for the corresponding objc version like this:

https://github.com/search?q=objc+779.1 https://github.com/LGCooci/objc4_debug

Summary

From now on you can boldly use C++ structs to store Objective-C objects.

---

Pretty interesting :)

文章目录

• 参考
• 总结

在使用Objective C++时，可以用C++的struct或者class存储Objective C对象。

例如：

struct CppStruct {
    MyObject *obj;
};

那么突然想，如果CppStruct析构了，MyObject会dealloc吗？不用多想，肯定也会。但如何做到的呢？

写段代码验证下，

@interface MyObject : NSObject
@end
@implementation MyObject
- (instancetype)init {
    self = [super init];
    if (self) {
        NSLog(@"MyObject init");
    }
    return self;
}
- (void)dealloc {
    NSLog(@"MyObject dealloc");
}
@end

struct CppStruct {
    MyObject *obj;
    CppStruct() {
        NSLog(@"CppStruct constructor");
    }
    ~CppStruct() {
        NSLog(@"CppStruct destructor");
    }
};

CppStruct * CreateStruct() {
    CppStruct * s = new CppStruct();
    s->obj = [[MyObject alloc] init];
    return s;
}

void FreeStruct(CppStruct *s) {
    delete s;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        CppStruct *s = CreateStruct();
        FreeStruct(s);
    }
    return 0;
}

如下输出

可知，MyObject确实dealloc了。

那么，断点到dealloc，看看怎么调用到的。

看到上图有两个 ~CppStruct()，其中一个调用了 objc_storeStrong，objc_storeStrong 进一步触发了MyObject的dealloc。

从上图可知，一定是编译器生成了objc_storeStrong调用的代码。

看下Disassembly，确实生成了objc_storeStrong调用代码。

进一步实锤。

LLVM 怎么去生成的这个逻辑，就不去翻了哈。

参考

上面调试中用了可编译的objc runtime，GitHub上有很多，可以如下搜索对应objc版本即可：

https://github.com/search?q=objc+779.1 https://github.com/LGCooci/objc4_debug

总结

以后可以大胆的用C++ struct 存储Objective C对象了。

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很有趣 :)