The implementation of object and value types in V8 is not necessarily as pointers versus data. Almost all objects and values are HeapObjects, and all internal references to them are through pointers to them. Objects are mutable HeapObjects, and values are immutable HeapObjects. When a value is replaced by another value, the pointer to the first value is replaced by a pointer to a new value with different contents. The old value object may still be around, if other variables point to it.
Sometimes a value is wrapped in a JSValue object, which gives it more of the properties of a object. There are some values that are not stored as HeapObjects. Some number values (small integers) are represented by 32-bit tagged non-pointer words, stored where a pointer to a HeapObject would normally be. On Thu, Feb 19, 2009 at 4:27 PM, Jan de Mooij <[email protected]> wrote: > Thanks for your reply. > > I understand JavaScript value/reference semantics though, but not the > implementation of it in V8. > I was wondering where the difference between value and reference types is > made. I suppose reference-types (JavaScript object and arrays) are > implemented as a reference-to-object whereas primitives directly contain the > value, but I couldn't find this anywhere. For example: in objects.h > HeapNumber and Array both derive from HeapObject, but HeapNumber is a > value-type (I suppose) whereas Array is a reference type. > > Thanks, > Jan > > > On Thu, Feb 19, 2009 at 4:00 PM, William Hesse <[email protected]>wrote: > >> JavaScript objects, primitive values, and properties are all very >> different, and should >> be explained well in good JavaScript books. The simplest way to achieve >> what you >> want (two variables aliased to the same information) is: >> >> var a = new Object(); >> var b = a >> // a and b point to the same object. That object's properties can be >> created and changed. >> a.x = 3; >> // b.x == 3 and a.x == 3 >> b.x = 5; >> // b.x == 5 and a.x == 5 >> >> Only a property of an object can be changed without changing the object's >> identity. >> Two variables or properties can point to the same object. >> >> Hope this helps, >> Bill Hesse >> >> >> On Wed, Feb 18, 2009 at 11:08 PM, Jan de M. <[email protected]> wrote: >> >>> >>> Hello, >>> >>> First, thanks for creating this great engine. I am evaluating using it >>> for one of my projects, so I started reading the source and I could >>> figure out most things myself, but I still don't quite understand how >>> variables, values and objects relate to each other. See this code, for >>> example: >>> >>> var a = 0; >>> var b = 1; >>> >>> This creates two locals, but is there any mapping from variable to >>> memory address? I found the SymbolTable class but I don't see how it >>> is used here. For example, would it be possible to let a point to the >>> exact same memory location (object) as b? >>> >>> Any help would be appreciated. >>> >>> Jan >>> >>> >> >> >> -- >> We can IMAGINE what is not >> >> >> >> > -- We can IMAGINE what is not --~--~---------~--~----~------------~-------~--~----~ v8-dev mailing list [email protected] http://groups.google.com/group/v8-dev -~----------~----~----~----~------~----~------~--~---
