[xsd-users] Default values for optional elements.

[Boris Kolpackov]

Hi David,

[I've CC'ed xsd-users back in.]


Moss, David R (SELEX Comms) (UK Christchurch) <david.r.moss at selex-comm.com> writes:

> I've had a play with the new version and noticed that the
> default_value() function creates and returns a copy each time it is
> called.
>
> This means that a function that does:
>
> return foo.bar ().present ? foo.bar ().get () : foo.bar::default_value
> ();
>
> has to return a copy. Whereas the following would not require any copies
> to be made and the client also would not require its own copy:
>
>
> static const type& default_value();
>
> inline
> const foo::bar::type& foo::bar::
> default_value ()
> {
>    static foo::bar::type  instance =
>    ::xsd::cxx::tree::default_value_factory<
> 		foo::bar::type>::create("default_value", 0);
>
>    return instance;
> }
>
> Any thoughts?


Hm, this is an interesting idea. I agree that it would be nice to
have return types of get() and default_value() aligned. However,
adding a static variable in the default_value will effectively make
it uninlinable. I guess it is a good tradeoff for string-based types
(where, currently, we are looking at heap allocation every time
default_value is called) but could be not so good for fundamental
C++ types (e.g., bool, int). However, this case can be specialized
to return by value (along with get(), which is on my TODO).

Yet another approach would be to have a class-static variable with
default value. The drawback is that it will always be initialized
no matter whether it is used or not. The advantages of this approach
include inlinable default_value and, more importantly, this makes it
usable in MT environments. This approach can also benefit from
specialization for fundamental types.

It seems to me like the class-static variable is the winner. Thoughts?


thanks,
-boris
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