理解谷歌的V8c++代码库中的继承-UnderstandinginheritanceinGoogle'sV8C++codebase

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The classes in objects.h do not actually define real C++ classes. They do not have any fields. The classes are merely facades to objects managed on the V8 Javascript heap. Hence they cannot have any virtual functions either, because that would require putting vtable pointers into the JS heap. Instead, all dispatch is done manual, via explicit type checks and down casts.

类的对象。h实际上没有定义真正的c++类。它们没有任何字段。这些类只是V8 Javascript堆上管理的对象的外观。因此它们也不能有任何虚函数，因为这需要将vtable指针放入到JS堆中。相反，所有的分派都是手工完成的，通过显式类型检查和下拉类型转换。

The this pointer inside methods isn't real either. For smis, this is simply an integer. For everything else it is a pointer into the V8 heap, off by one for tagging. Any actual accessor method masks this pointer and adds an offset to access the appropriate address in the heap. The offsets of each field is also defined manually in the classes.

方法中的这个指针也不是真实的。对于smis，这只是一个整数。对于其他一切，它是一个指向V8堆的指针，一个用于标记。任何实际的访问器方法都会掩盖这个指针，并添加一个偏移量来访问堆中的适当地址。每个字段的偏移量也在类中手工定义。

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Take a look at Object::IsPromise() for a perfect example of how it works:

看看Object::IsPromise()，它是如何工作的一个完美例子:

bool Object::IsPromise(Handle object) {
  if (!object->IsJSObject()) return false;
  auto js_object = Handle::cast(object);
  // Promises can't have access checks.
  if (js_object->map()->is_access_check_needed()) return false;
  auto isolate = js_object->GetIsolate();
  // TODO(dcarney): this should just be read from the symbol registry so as not
  // to be context dependent.
  auto key = isolate->promise_status();
  // Shouldn't be possible to throw here.
  return JSObject::HasRealNamedProperty(js_object, key).FromJust();
}

The way inheritance is used here is static. That is, type queries are done by a proxy or container (using some hidden magic, that, at a glance looks like they're using references to query a tag), and conversions from Object to a derived class is done by static_cast<>(). In that way, the member functions of the derived class can be called.

这里使用继承的方式是静态的。即类型查询是通过代理或容器(使用一些隐藏的魔法,乍一看像他们使用引用来查询一个标签),和转换从一个派生类对象是通过static_cast <>()。这样的话,派生类的成员函数可以调用。

Note that in the above function, the type query and cast is indirectly performed by the Handle<> class, not by Object or any of its derived classes.

注意，在上面的函数中，类型查询和转换是由句柄<>类间接执行的，而不是由对象或它的任何派生类执行的。

Note also that the functions which accept ObjectVisitor as a parameter are rather uniformly called Iterate, and that these functions all appear on proxies or handles.

还请注意，接受ObjectVisitor作为参数的函数相当一致地称为Iterate，并且这些函数都出现在代理或句柄上。