OC对象原理探究

1.alloc 方法到底干了什么?

---

先来看一段代码

Person *p1 = [Person alloc]; 
Person *p2 = [p1 init];
Person *p3 = [p1 init];
NSLog(@"%@ --- %p",p1,&p1);
NSLog(@"%@ --- %p",p2,&p2);
NSLog(@"%@ --- %p",p3,&p3);

<Person: 0x10103be00> --- 0x7ffeefbff4b0
<Person: 0x10103be00> --- 0x7ffeefbff4a0
<Person: 0x10103be00> --- 0x7ffeefbff4a8

• 这里通过打印结果发现 p1, p2, p3 的内存地址是相同的说明它们指向的是同一堆区内存区域,但是指针地址不同

• 如图所示:

  
  内存分布.png

2.从 OC 源码的跟踪 alloc

---

从源码直接 command + click 查看 alloc 方法

+ (id)alloc {
    return _objc_rootAlloc(self);
}

在 _objc_rootAlloc 方法里面调用了 callAlloc方法

id  _objc_rootAlloc(Class cls) {
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

在 callAlloc方法里面调用了_class_createInstanceFromZone

// Call [cls alloc] or [cls allocWithZone:nil], with appropriate 
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
    if (slowpath(checkNil && !cls)) return nil;
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil);
    }
#endif

    // No shortcuts available.
    if (allocWithZone) {
        return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
    }
    return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}

这里面调用了_class_createInstanceFromZone方法

NEVER_INLINE
id  _objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
    // allocWithZone under __OBJC2__ ignores the zone parameter
    return _class_createInstanceFromZone(cls, 0, nil,
                                         OBJECT_CONSTRUCT_CALL_BADALLOC);
}

最关键就在_class_createInstanceFromZone方法里面

static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
                              int construct_flags = OBJECT_CONSTRUCT_NONE,
                              bool cxxConstruct = true,
                              size_t *outAllocatedSize = nil)
{
    ASSERT(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;
    // 1:要开辟多少内存
    size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
        // 2;怎么去申请内存
        obj = (id)calloc(1, size);
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    // 3: 初始化 isa 关联类?
    if (!zone && fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else {
        // Use raw pointer isa on the assumption that they might be
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

    if (fastpath(!hasCxxCtor)) {
        return obj;
    }

    construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
    return object_cxxConstructFromClass(obj, cls, construct_flags);
}

这里面主要做三件事情

1.计算需要开辟的内存空间大小 size = cls->instanceSize(extraBytes);
2.开辟内存 obj = (id)calloc(1, size);
3.实例化 isa obj->initIsa(cls);

下面是总结的alloc方法的实际流程图

流程图.png

3.发现问题

---

1.为什么一开始我们调用的 alloc 的方法，底层却调用的是 objc_alloc

这里是因为在 llvm 源码中系统在编译期间就对 alloc 方法做了替换