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clean up
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√(noham)²
@@ -1,157 +0,0 @@
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# Objective-C 底层
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1. Objective-C 中对象、类主要是基于 C/C++ 中的结构体实现的。
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方法一：
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可以用 clang 验证。`clang -rewrite-objc main.m -o main.cpp`
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转到指定平台代码。`xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp`
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```
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struct NSObject_IMPL {
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Class isa;
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};
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/// An opaque type that represents an Objective-C class.
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typedef struct objc_class *Class;
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```
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所以 Class 是指向结构体的指针，所以在 64 位系统中占据8个字节，32位系统占据4个字节。
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NSObject 就是结构体占据8个字节。
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发现 `class_getInstanceSize` 和 `malloc_size` 结果不一致。
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```c
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// Class's ivar size rounded up to a pointer-size boundary.
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uint32_t alignedInstanceSize() const {
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return word_align(unalignedInstanceSize());
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}
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```
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`class_getInstanceSize` 返回的是类的实例对象的成员变量的大小（四舍五入等于指针指向对象的大小，所以不精确）
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```c
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extern size_t malloc_size(const void *ptr);
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/* Returns size of given ptr */
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```
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`malloc_size` 返回的就是指针指向的对象大小.
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结论：
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- 当某个类继承自 NSObject 的时候，如果没有其他属性，则这个类占据16个字节。`class_getInstanceSize` 占据 8 ， `malloc_size` 占据 16
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- 当某个类继承自 NSObject 的时候，如果有其他属性，则这个类占据16个字节。`class_getInstanceSize` 占据 16 ， `malloc_size` 占据 16
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方法二： 从源代码角度出发验证(从上到下)
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```c++
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// NSObject.mm
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// Replaced by ObjectAlloc
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+ (id)allocWithZone:(struct _NSZone *)zone {
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return _objc_rootAllocWithZone(self, (malloc_zone_t *)zone);
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}
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// objc-class-old.mm
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id
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_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone)
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{
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id obj;
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if (fastpath(!zone)) {
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obj = class_createInstance(cls, 0);
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} else {
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obj = class_createInstanceFromZone(cls, 0, zone);
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}
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if (slowpath(!obj)) obj = _objc_callBadAllocHandler(cls);
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return obj;
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}
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// objc-class-old.mm
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/***********************************************************************
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* _class_createInstance. Allocate an instance of the specified
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* class with the specified number of bytes for indexed variables, in
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* the default zone, using _class_createInstanceFromZone.
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**********************************************************************/
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static id _class_createInstance(Class cls, size_t extraBytes)
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{
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return _class_createInstanceFromZone (cls, extraBytes, nil);
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}
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static ALWAYS_INLINE id
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_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
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int construct_flags = OBJECT_CONSTRUCT_NONE,
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bool cxxConstruct = true,
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size_t *outAllocatedSize = nil)
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{
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ASSERT(cls->isRealized());
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// Read class's info bits all at once for performance
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bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
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bool hasCxxDtor = cls->hasCxxDtor();
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bool fast = cls->canAllocNonpointer();
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size_t size;
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size = cls->instanceSize(extraBytes);
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if (outAllocatedSize) *outAllocatedSize = size;
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id obj;
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if (zone) {
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obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
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} else {
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obj = (id)calloc(1, size);
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}
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if (slowpath(!obj)) {
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if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
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return _objc_callBadAllocHandler(cls);
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}
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return nil;
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}
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if (!zone && fast) {
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obj->initInstanceIsa(cls, hasCxxDtor);
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} else {
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// Use raw pointer isa on the assumption that they might be
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// doing something weird with the zone or RR.
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obj->initIsa(cls);
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}
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if (fastpath(!hasCxxCtor)) {
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return obj;
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}
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construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
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return object_cxxConstructFromClass(obj, cls, construct_flags);
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}
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// objc-runtime-new.h
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// Class's ivar size rounded up to a pointer-size boundary.
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uint32_t alignedInstanceSize() const {
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return word_align(unalignedInstanceSize());
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}
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// objc-runtime-new.h
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size_t instanceSize(size_t extraBytes) const {
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if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
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return cache.fastInstanceSize(extraBytes);
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}
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size_t size = alignedInstanceSize() + extraBytes;
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// CF requires all objects be at least 16 bytes.
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if (size < 16) size = 16;
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return size;
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}
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```
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`CF requires all objects be at least 16 bytes.` 系统为 NSObject 对象分配了至少16个字节大小的空间，但是它使用了 8 个字节大小的空间用来存放 ivars（64位系统）
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2. 某个类继承自 NSObject 的情况，内存如何分配
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