通过[[XXXX alloc] init]创建实例对象我们再熟悉不过了，今天我们来探索一下alloc的底层原理，我们先写下面一行代码，打上断点并执行。
JPerson *p = [JPerson alloc];
复制代码按住control点击step into可以看到符号objc_alloc，我们设置符号断点objc_alloc可以看到objc_alloc是在libobjc.A.dylib中
libobjc.A.dylib`objc_alloc
复制代码这里使用的是objc4-781.2版本
objc_alloc
// Calls [cls alloc].
id
objc_alloc(Class cls)
{
return callAlloc(cls, true/*checkNil*/, false/*allocWithZone*/);
}
复制代码callAlloc
// 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__
// 目前我们使用的是objc2版本，看这里
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));
}
复制代码目前我们使用的都是objc2版本，所以先看_objc_rootAllocWithZone。
hasCustomAWZ()是否有自定义的allocWithZone方法，我们后面研究isa的时候再来探索
bool hasCustomAWZ() const {
return !cache.getBit(FAST_CACHE_HAS_DEFAULT_AWZ);
}
复制代码_objc_rootAllocWithZone
NEVER_INLINE
id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
// allocWithZone under __OBJC2__ ignores the zone parameter
// objc2版本中忽略了zone参数
return _class_createInstanceFromZone(cls, 0, nil,OBJECT_CONSTRUCT_CALL_BADALLOC);
}
复制代码_class_createInstanceFromZone
/***********************************************************************
* class_createInstance
* fixme
* Locking: none
*
* Note: this function has been carefully written so that the fastpath
* takes no branch.
**********************************************************************/
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)
{
// 避免多线程问题，这里同样涉及到isa
ASSERT(cls->isRealized());
// Read class's info bits all at once for performance
// cls及其父类是否有c++构造函数和析构函数
bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
bool hasCxxDtor = cls->hasCxxDtor();
// 实例对象的isa是一个单纯的指针true，isa不是一个纯粹的指针为false
// 后面研究isa的之后还会涉及到
bool fast = cls->canAllocNonpointer();
size_t size;
// 获取需要开辟的空间大小
size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
if (zone) { // zone为nil
obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
obj = (id)calloc(1, size);
}
// 处理obj空间开辟失败的情况
if (slowpath(!obj)) {
if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
return _objc_callBadAllocHandler(cls);
}
return nil;
}
// objc2版本中canAllocNonpointer()=true
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);
}
复制代码这个方法实现了：获取要开辟空间的大小，并开辟空间，初始化isa然后返回指针
instanceSize
size_t instanceSize(size_t extraBytes) const {
// 是否缓存过需要开辟的空间大小
if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
return cache.fastInstanceSize(extraBytes);
}
// 8字节对齐，如果小于16字节，那么返回16字节
size_t size = alignedInstanceSize() + extraBytes;
// CF requires all objects be at least 16 bytes.
if (size < 16) size = 16;
return size;
}
复制代码fastInstanceSize
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);
}
}
复制代码调用了align16
static inline size_t align16(size_t x) {
return (x + size_t(15)) & ~size_t(15);
}
复制代码将x加上15以后低四位清零，即16字节对齐
initInstanceIsa
inline void
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
ASSERT(!cls->instancesRequireRawIsa());
ASSERT(hasCxxDtor == cls->hasCxxDtor());
initIsa(cls, true, hasCxxDtor);
}
inline void
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor)
{
// 不能是taggetPointer对象
ASSERT(!isTaggedPointer());
if (!nonpointer) {
isa = isa_t((uintptr_t)cls);
} else {
ASSERT(!DisableNonpointerIsa);
ASSERT(!cls->instancesRequireRawIsa());
isa_t newisa(0);
#if SUPPORT_INDEXED_ISA
ASSERT(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.shiftcls = (uintptr_t)cls >> 3;
#endif
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}
}
复制代码这里创建了isa然后赋上了一些值，具体这些值是什么意思我们分析isa指针的时候再详细介绍
init
- (id)init {
return _objc_rootInit(self);
}
id
_objc_rootInit(id obj)
{
// In practice, it will be hard to rely on this function.
// Many classes do not properly chain -init calls.
return obj;
}
+ (id)new {
return [callAlloc(self, false/*checkNil*/) init];
}
复制代码init其实什么都没有实现，只是为开发者提供了一个便捷的接口，new其实是alloc和init连着写的写法
本文中涉及到了很多isa相关的操作，我们将会在下一篇文章中详细讨论isa