这篇文章主要介绍“js引擎HeapObject怎么实现”，在日常操作中，相信很多人在js引擎HeapObject怎么实现问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”js引擎HeapObject怎么实现”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！

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v8的对象基类是Object。我们先看一下他的类定义。下面只列出重要的函数。

   
     
 
    
   
// Object is the abstract superclass for all classes in the
// object hierarchy.
// Object does not use any virtual functions to avoid the
// allocation of the C++ vtable.
// Since Smi and Failure are subclasses of Object no
// data members can be present in Object.
class Object BASE_EMBEDDED {
public:
 // Type testing.
 inline bool IsSmi();
 // 下面是一些列isXX的函数
 // Extract the number.
 inline double Number();

 Object* ToObject();             // ECMA-262 9.9.
 Object* ToBoolean();            // ECMA-262 9.2.

 // Convert to a JSObject if needed.
 // global_context is used when creating wrapper object.
 Object* ToObject(Context* global_context);

 // Converts this to a Smi if possible.
 // Failure is returned otherwise.
 inline Object* ToSmi();

 void Lookup(String* name, LookupResult* result);

 // Property access.
 inline Object* GetProperty(String* key);
 inline Object* GetProperty(String* key, PropertyAttributes* attributes);
 Object* GetPropertyWithReceiver(Object* receiver,
                                 String* key,
                                 PropertyAttributes* attributes);
 Object* GetProperty(Object* receiver,
                     LookupResult* result,
                     String* key,
                     PropertyAttributes* attributes);
 Object* GetPropertyWithCallback(Object* receiver,
                                 Object* structure,
                                 String* name,
                                 Object* holder);

 inline Object* GetElement(uint32_t index);
 Object* GetElementWithReceiver(Object* receiver, uint32_t index);

 // Return the object's prototype (might be Heap::null_value()).
 Object* GetPrototype();

 // Returns true if this is a JSValue containing a string and the index is
 // < the length of the string.  Used to implement [] on strings.
 inline bool IsStringObjectWithCharacterAt(uint32_t index);

 // Casting: This cast is only needed to satisfy macros in objects-inl.h.
 static Object* cast(Object* value) { return value; }

 // Layout description.
 static const int kSize = 0;  // Object does not take up any space.

private:
 // 禁止对象在堆中创建
 /*
     宏展开是
     Object();
     Object(const TypeName&);
     void operator=(const Object&)
 */
 DISALLOW_IMPLICIT_CONSTRUCTORS(Object);
};


   
     
 
    
    

我们看到类中有一个静态属性kSize，这个属性是标记该类的对象，属性需要占据的内存字节大小。下面我们看第一个继承于Object的类Smi。Smi是表示小整形。我们看他的定义。

   
     
 
    
   
class Smi: public Object {
public:
 // Returns the integer value.
 inline int value();

 // Convert a value to a Smi object.
 static inline Smi* FromInt(int value);

 // Returns whether value can be represented in a Smi.
 static inline bool IsValid(int value);

 // Casting.
 static inline Smi* cast(Object* object);
 // Min and max limits for Smi values.
 static const int kMinValue = -(1 << (kBitsPerPointer - (kSmiTagSize + 1)));
 static const int kMaxValue = (1 << (kBitsPerPointer - (kSmiTagSize + 1))) - 1;

private:
 DISALLOW_IMPLICIT_CONSTRUCTORS(Smi);
};


   
     
 
    
    

Smi的实现。主要有两个函数。

   
     
 
    
   
int Smi::value() {
 return reinterpret_cast(this) >> kSmiTagSize;
}


Smi* Smi::FromInt(int value) {
 ASSERT(Smi::IsValid(value));
 // kSmiTagSize是类型标记，表示是小整形。值是1.kSmiTag是0
 return reinterpret_cast((value << kSmiTagSize) | kSmiTag);
}


   
     
 
    
    

我们看到Smi的实现比较简单。我们接着看继承关系中的下一个类HeapObject。HeapObject类是表示他的对象是在堆中分配内存的。下面是类定义。

   
     
 
    
   
// HeapObject is the superclass for all classes describing heap allocated
// objects.
class HeapObject: public Object {
public:
 // [map]: contains a Map which contains the objects reflective information.
 inline Map* map();
 inline void set_map(Map* value);

 // Converts an address to a HeapObject pointer.
 // 对象的地址+对象标记
 static inline HeapObject* FromAddress(Address address);

 // Returns the address of this HeapObject.
 // 对象的真正地址
 inline Address address();

 // Iterates over pointers contained in the object (including the Map)
 void Iterate(ObjectVisitor* v);

 // Iterates over all pointers contained in the object except the
 // first map pointer.  The object type is given in the first
 // parameter. This function does not access the map pointer in the
 // object, and so is safe to call while the map pointer is modified.
 void IterateBody(InstanceType type, int object_size, ObjectVisitor* v);

 // This method only applies to struct objects.  Iterates over all the fields
 // of this struct.
 void IterateStructBody(int object_size, ObjectVisitor* v);

 // Copy the body from the 'from' object to this.
 // Please note the two object must have the same map prior to the call.
 inline void CopyBody(JSObject* from);

 // Returns the heap object's size in bytes
 inline int Size();

 // Given a heap object's map pointer, returns the heap size in bytes
 // Useful when the map pointer field is used for other purposes.
 // GC internal.
 inline int SizeFromMap(Map* map);

 static inline Object* GetHeapObjectField(HeapObject* obj, int index);

 // Casting.
 static inline HeapObject* cast(Object* obj);

 // Dispatched behavior.
 void HeapObjectShortPrint(StringStream* accumulator);

 // Layout description.
 // First field in a heap object is map.
 static const int kMapOffset = Object::kSize;
 static const int kSize = kMapOffset + kPointerSize;

protected:
 // helpers for calling an ObjectVisitor to iterate over pointers in the
 // half-open range [start, end) specified as integer offsets
 inline void IteratePointers(ObjectVisitor* v, int start, int end);
 // as above, for the single element at "offset"
 inline void IteratePointer(ObjectVisitor* v, int offset);

 // Computes the object size from the map.
 // Should only be used from SizeFromMap.
 int SlowSizeFromMap(Map* map);

private:
 DISALLOW_IMPLICIT_CONSTRUCTORS(HeapObject);
};


   
     
 
    
    

我们先看一下HeapObject类的对象的内存布局。

   
     
 
    
   
static const int kMapOffset = Object::kSize; // 0
static const int kSize = kMapOffset + kPointerSize; // kPointerSize表示一个指针变量的大小


   
     
 
    
    

下面我们开始HeapObject的实现。从之前的分析我们知道，v8很多对象的属性不是和传统的C++那样，直接定义一个类型的。而且通过给属性分配字节数去控制的。所以分析之前我们要先了解一个东西，就是如何读写对象的一个属性。

   
     
 
    
   
// 获取对象某个属性的地址，p是对象的首地址，offset是偏移，kHeapObjectTag是对象的标记，算地址的时候需要减掉
#define FIELD_ADDR(p, offset) \
 (reinterpret_cast(p) + offset - kHeapObjectTag)

// 读取对象中某个属性的值，指向对象地址空间的某个地址，转成对象指针
#define READ_FIELD(p, offset) \
 (*reinterpret_cast(FIELD_ADDR(p, offset)))

// 给对象的某个属性赋值
#define WRITE_FIELD(p, offset, value) \
 (*reinterpret_cast(FIELD_ADDR(p, offset)) = value)


   
     
 
    
    

然后我们接着看HeapObject的实现。

   
     
 
    
   
// 堆对象的开始地址是一个Map对象
Map* HeapObject::map() {
 return reinterpret_cast READ_FIELD(this, kMapOffset);
}

// 设置堆对象的map对象
void HeapObject::set_map(Map* value) {
 WRITE_FIELD(this, kMapOffset, value);
}


   
     
 
    
    

上面就是读写对象的某个属性的例子(heapObject只有一个map属性)。首先根据属性在对象内存布局中的偏移找到属性的地址，然后把他转成Object对象（基类），然后把value写进去，这里是一个Map对象。读取的时候也是先转成Object对象。然后再转成Map对象。map属性在所有对象中都是在第一个位置。

   
     
 
    
   
// 封装过的地址,kHeapObjectTag表示是一个堆对象
HeapObject* HeapObject::FromAddress(Address address) {
 ASSERT_TAG_ALIGNED(address);
 return reinterpret_cast(address + kHeapObjectTag);
}

// 对象的真正地址
Address HeapObject::address() {
 return reinterpret_cast
(this) - kHeapObjectTag;
}


   
     
 
    
    

上面是对对象地址的封装，低一位表示类型。即堆对象。

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