- From: Jeni Tennison <jeni@jenitennison.com>
- Date: Tue, 27 Aug 2002 23:02:44 +0100
- To: xmlschema-dev@w3.org, Stefan Wachter <Stefan.Wachter@gmx.de>
Hi Stefan, > I thought that a fixed value is also a default value. No. Both fixed and default values are what's termed "value constraints", but they are distinct types of value constraints. They both have the semantic that if an attribute then the value (fixed or default) gets used, but with a default value the value of the attribute can be specified as something different in the instance whereas with a fixed value it can't -- if the attribute is specified then it *must* be given the fixed value. A default value constraint therefore only comes into play when the attribute is missing (which is why you're not allowed to give a default for a required attribute -- it would never come into play). A fixed value constraint, on the other hand, has an influence on what values can be specified for an attribute, so has an effect when the attribute is present as well as when it's absent. It's therefore perfectly rationale to have an attribute that's both required (has to be present in the instance) and fixed (has to have a particular value). Cheers, Jeni --- Jeni Tennison http://www.jenitennison.com/
Received on Tuesday, 27 August 2002 18:02:45 UTC