1.alloc 方法到底干了什么?

先來看一段代碼

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

• 這里通過打印結(jié)果發(fā)現(xiàn) p1, p2, p3 的內(nèi)存地址是相同的說明它們指向的是同一堆區(qū)內(nèi)存區(qū)域,但是指針地址不同

• 如圖所示:

  
  
  內(nèi)存分布.png

2.從 OC 源碼的跟蹤 alloc

從源碼直接 command + click 查看 alloc 方法

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

在 _objc_rootAlloc 方法里面調(diào)用了 callAlloc方法

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

在 callAlloc方法里面調(diào)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));
}

這里面調(diào)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);
}

最關(guān)鍵就在_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:要開辟多少內(nèi)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;怎么去申請內(nèi)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 關(guān)聯(lián)類?
    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);
}

這里面主要做三件事情

1.計算需要開辟的內(nèi)存空間大小 size = cls->instanceSize(extraBytes);
2.開辟內(nèi)存 obj = (id)calloc(1, size);
3.實例化 isa obj->initIsa(cls);

下面是總結(jié)的alloc方法的實際流程圖

流程圖.png

3.發(fā)現(xiàn)問題

1.為什么一開始我們調(diào)用的 alloc 的方法驴娃，底層卻調(diào)用的是 objc_alloc

這里是因為在 llvm 源碼中系統(tǒng)在編譯期間就對 alloc 方法做了替換