Subj : Re: Performance Interfacing to C++
To   : netscape.public.mozilla.jseng
From : Thorsten R
Date : Sat Aug 28 2004 03:04 am

> You can use JSPROP_SHARED to avoid a value slot for such properties, but
> the engine will still build a hash table per object map (per object with
> its own properties, not one delegating all properties to its prototype),
> and those hash table entries will point into a global (per-runtime)
> lexicographic tree.

Can you give me a hint how to create an object which delegates all
properties to its prototype? Or is this the default as long as I don't add
any individual properties to an object?

I'm currently using JS_InitClass() to create my prototype and then
JS_DefineObject() for each instance of my internal C++ objects. I do not add
any additional properties or methods to individual instances. (BTW, I found
out that using a prototype requires about half as much memory than calling
JS_DefineObject() without a prototype.)

> Strings require special handling even if you store references to them in
> your private (to the JS engine) data and do not use value slots (i.e.,
> use JSPROP_SHARED as suggested above), if you want to avoid JS taking
> ownership of the malloc'd jschar array, or copying from your storage
> into its own.  See JS_AddExternalStringFinalizer and its comments in
> jsapi.h.

Thank you for pointing me to this function. I think I have to call
JS_NewExternalString() in my C++ getter to return an internal C++ string to
the JS engine, is this correct? If my finalizer callback function is called,
do I *need* to free the string? I don't think so. And I can't do so, because
it is freed in the destructor of my C++ object. Is it possible to call
JS_AddExternalStringFinalizer() with finalizer = NULL, so I just get my
type-code?

There comes one additional question to my mind:
Let's assume I have a C++ class "Text" with a member char *m_szStr, which is
"exported" to the JS engine as the property "Text.Str".

Then I execute the following JS code:
Text.Str = "Hello World";
Text.Str = "xyz";

If the C++ setter is called the second time by the engine due to the
assignment (Text.Str = "xyz"), is it safe to free my internal C++ string at
that moment? Or am I not allowed to do so, because the GC or anything else
in SpiderMonkey is still working with it, and I must wait until my
StringFinalizer callback is called?

Regards
Thorsten

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