開發(fā)中,我們經(jīng)常使用到[[xxx alloc] init],但卻沒有真正理解過alloc,init,現(xiàn)在進(jìn)行深入了解一下搓扯。

alloc,init的功能

1.先看看這樣的一個(gè)例子

//創(chuàng)建一個(gè)Person類,然后引用對(duì)應(yīng)頭文件
        Person *p1 = [Person alloc];
        Person *p2 = [p1 init];
        Person *p3 = [p1 init];
        
        NSLog(@"%p-%p", p1, &p1);
        NSLog(@"%p-%p", p2, &p2);
        NSLog(@"%p-%p", p3, &p3);

運(yùn)行結(jié)果

• p1,p2,p3的指向地址都相同
• p1,p2,p3的地址不同

2.alloc與init的關(guān)系如圖

• alloc生成對(duì)象地址礁芦。
• init分配指針地址,并指向?qū)ο蟮刂?/li>

alloc流程

需要看到alloc流程,可到蘋果開源網(wǎng)站下載。
如果要將源碼進(jìn)行編譯調(diào)試,可參見

• https://juejin.im/post/6844903959161733133
• https://juejin.im/post/6844904082226806792

對(duì)源碼進(jìn)行分析

從alloc方法一步一步點(diǎn)進(jìn)去我們分別能看到：

//NSObject.mm文件中  +alloc方法
+ (id)alloc {
    return _objc_rootAlloc(self);
}

//NSObject.mm文件中  _objc_rootAlloc(Class cls)方法
id _objc_rootAlloc(Class cls)
{
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

//NSObject.mm文件中  callAlloc方法 
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));
}

//objc-runtime-new.mm文件中  _objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)方法 
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);
}

//objc-runtime-new.mm文件中 _class_createInstanceFromZone方法 
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;

    size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
        obj = (id)calloc(1, size);
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    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);
}

1. 前幾步一條路進(jìn)入可以不提棍矛，在callAlloc方法中蕴掏，出現(xiàn)了分支slowpath和fastpath门坷，快速路徑與慢速路徑默勾，可以理解為大多數(shù)情況走快速路徑，極少情況走慢速路徑乍恐，這樣寫便于運(yùn)行效率的提升评疗，所以我們繼續(xù)研究_objc_rootAllocWithZone方法。

2. 緊接著我們到了關(guān)鍵的_class_createInstanceFromZone方法：

1. _class_createInstanceFromZone第一個(gè)研究的方法size = cls->instanceSize(extraBytes);

    inline size_t instanceSize(size_t extraBytes) const {
        if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
            return cache.fastInstanceSize(extraBytes);
        }

        size_t size = alignedInstanceSize() + extraBytes;
        // CF requires all objects be at least 16 bytes.
        if (size < 16) size = 16;
        return size;
    }

    size_t fastInstanceSize(size_t extra) const
    {
        ASSERT(hasFastInstanceSize(extra));

        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        } else {
            size_t size = _flags & FAST_CACHE_ALLOC_MASK;
            // remove the FAST_CACHE_ALLOC_DELTA16 that was added
            // by setFastInstanceSize
            return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);
        }
    }

經(jīng)過調(diào)試茵烈，我們能看到進(jìn)入fastInstanceSize方法壤巷，然后執(zhí)行return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);

static inline size_t align16(size_t x) {
    return (x + size_t(15)) & ~size_t(15);
}

很明顯，這是在進(jìn)行內(nèi)存對(duì)齊（下一章節(jié)具體講解）

3. 得到一個(gè)16字節(jié)倍數(shù)大小size瞧毙，然后執(zhí)行obj = (id)calloc(1, size);分配內(nèi)存空間（ps:if (zone)現(xiàn)已廢棄胧华，不做過深研究）

當(dāng)我們?cè)趫?zhí)行obj = (id)calloc(1, size);后我們答應(yīng)obj可以看到：

4.當(dāng)我們執(zhí)行完obj->initInstanceIsa(cls, hasCxxDtor);我們拿到我們想要的結(jié)果:

綜上alloc運(yùn)行流程如圖: