Duktape has no Spidermonkey equivalent of JS_SetPrivate() about duktape HOT 7 CLOSED

The duk_get_prop() call has the following stack signature:

[ ... obj! ... key! ] -> [ ... obj! ... val! ]

The argument index identifies the object on the stack. The topmost value on the stack is the key being looked up from the object, and gets replaced by the property value.

Your duk_get_prop() call is probably incorrect:

duk_push_this(ctx);
duk_get_prop(ctx, -1);

You're using the same value as both the object and the key, essentially reading the property:

this[this]

The key (this) is popped off the stack, and this[this] replaces it. At the end there is only one value on the stack, the lookup result.

What key do you want to look up from the this object? Can you give an Ecmascript equivalent of what you want to do?

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@svaarala Don't have key. because I should bind C++ Instance into JSObject( pointer ).
for example,
Student* std = new Student();
duk_push_object(ctx);
duk_push_pointer(std);
duk_put_prop(ctx,-2);
...(omit)...

and
static int student_callback(duk_context ctx) {
duk_push_this(ctx);
duk_get_prop(ctx,-1);
Student * std=(Student)duk_get_pointer(ctx, -1);
duk_push_int(std->age);
return 1;
}

from duktape.

If you're trying to store a pointer into an object you need to have a key to store it with, e.g. to store:

duk_push_object(ctx);
duk_push_pointer(ctx, (void *) std);
duk_put_prop_string(ctx, -2, "student");  /* obj.student = ptr */

and then to read it back (assuming this is bound to the object created above):

duk_push_this(ctx);
duk_get_prop_string(ctx, -1, "student");  /* pushes this.student */
Student *std = (Student *) duk_require_pointer(ctx, -1);
/* ... */

from duktape.

@svaarala Thank you for your answer & good solution :), but I have found API like JS_SetPrivate on the spider monkey.
Thank you :)

from duktape.

Duktape doesn't currently have an equivalent API concept, you can only store string keyed properties on an object.

I could add a convenience API to associate a void * with an object, but that would be implemented by storing a property on the object. For this, Duktape has a concept of "internal" property names which cannot occur in plain Ecmascript code, so they won't interfere with normal code (and won't enumerate etc).

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@svaarala Ok, Thank you & Thanks for duktape. :)

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Up to Duktape 0.11.0 the "internal" properties were only for Duktape's own internal use, but in 0.12.0 (and current master) they're exposed to user code too, as a way to make hidden properties not easily accessible from Ecmascript code.

Technically internal property names are simply normal strings which begin with the byte 0xFF. This byte makes the property name invalid UTF-8 so that ordinary keys from Ecmascript code cannot clash with the internal property names (normal key names are valid UTF-8). Here's an example:

https://github.com/svaarala/duktape/blob/master/api-testcases/test-dev-internal-property-basics.c

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