Subj : Re: JS_IsAssigning
To   : James Turner <jmtn@blueyonder.co.uk>
From : Brendan Eich <brendan@meer.net>
Date : Fri Aug 29 2003 11:24 am

James Turner wrote:
>>Why do you need to detect a difference when resolving or getting 'bar' 
>>in foo between 'baz = foo.bar.x;' and 'foo.bar.x = 42;'?
> 
> 
> Okay, this is going to take some explaining. Possibly, my basic design
> is a bit wonky.
> 
> In the example above, 'foo' is actually a JS object associated with a
> native (C++) class called 'GameEntity'. One of the things the C++
> class defines is a map of property names to jsvals, and this acts as a
> store for the various 'dynamic' (as opposed to hard-coded) props of
> the gameEntity.

[snip]

> Okay, now someone comes to access the 'hairyness' of 'cat'. In
> GameEntity's getter impl, the code essential does this:
> 
> std::string propName = .... from the 'id' jsval ;
> if (gameEntity->props.find(propName)) {
>   *rv = gameEntity->props[propName];
> } else {
>   // value wasn't locally set, use the default
>   *rv = gameEntity->archetpe->props[propName].defaultValue;
> }
> 
> This should hopefully reveal the problem. When the property type is an
> object, and the value has not been locally set on the GameEntity, I
> end up returning the defaultValue jsval, and then it gets modified -
> not the desired result!

Is the code above running for a get of foo.bar, or for a set of 
foo.bar.x?  If a get of foo.bar, then I don't see how the default value 
of foo.bar will be modified by setting foo.bar.x = 42.

I must be missing something.


  What I would like to do is determine if the
> jsval I'm returning from my getter is going to be changed, and if so,
> force it to be locally created on the GameEntity *before* I return it
> from the getter.

But setting foo.bar.x doesn't change foo.bar, so again (or still) I'm 
confused.

/be

.