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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