# Objective-C 底层

1. Objective-C 中对象、类主要是基于 C/C++ 中的结构体实现的。

  方法一：

  可以用 clang 验证。`clang -rewrite-objc main.m -o main.cpp`
  转到指定平台代码。`xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp`

  ``` 
  struct NSObject_IMPL {
  	Class isa;
  };
  
  /// An opaque type that represents an Objective-C class.
  typedef struct objc_class *Class;
  ```

  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。

  NSObject 就是结构体占据8个字节。

  ![image-20200726130625217](/Users/lbp/Library/Application Support/typora-user-images/image-20200726130625217.png)

  ![image-20200726125926892](/Users/lbp/Library/Application Support/typora-user-images/image-20200726125926892.png)

  发现 `class_getInstanceSize` 和 `malloc_size` 结果不一致。

  ```c
  // Class's ivar size rounded up to a pointer-size boundary.
  uint32_t alignedInstanceSize() const {
  	return word_align(unalignedInstanceSize());
  }
  ```

  `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）

  ```c
  extern size_t malloc_size(const void *ptr);
      /* Returns size of given ptr */
  ```

  `malloc_size` 返回的就是指针指向的对象大小.

  ![image-20200726130920234](/Users/lbp/Library/Application Support/typora-user-images/image-20200726130920234.png)

  结论：

  - 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
  - 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

  方法二： 从源代码角度出发验证(从上大下)

  ```c++
  // NSObject.mm
  // Replaced by ObjectAlloc
  + (id)allocWithZone:(struct _NSZone *)zone {
      return _objc_rootAllocWithZone(self, (malloc_zone_t *)zone);
  }
  
  // objc-class-old.mm
  id
  _objc_rootAllocWithZone(Class cls, malloc_zone_t *zone)
  {
      id obj;
  
      if (fastpath(!zone)) {
          obj = class_createInstance(cls, 0);
      } else {
          obj = class_createInstanceFromZone(cls, 0, zone);
      }
  
      if (slowpath(!obj)) obj = _objc_callBadAllocHandler(cls);
      return obj;
  }
  
  // objc-class-old.mm
  /***********************************************************************
  * _class_createInstance.  Allocate an instance of the specified
  * class with the specified number of bytes for indexed variables, in
  * the default zone, using _class_createInstanceFromZone.
  **********************************************************************/
  static id _class_createInstance(Class cls, size_t extraBytes)
  {
      return _class_createInstanceFromZone (cls, extraBytes, nil);
  }
  
  
  
  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);
  }
  
  // objc-runtime-new.h
  // Class's ivar size rounded up to a pointer-size boundary.
  uint32_t  alignedInstanceSize() const {
    return word_align(unalignedInstanceSize());
  }
  
  // objc-runtime-new.h
  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;
  }
  ```

  `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）

2. 某个类继承自 NSObject 的情况，内存如何分配