More than one invariant per struct/class

[simendsjo]

On 06.08.2011 22:53, Jonathan M Davis wrote:
> On Saturday 06 August 2011 22:40:15 simendsjo wrote:
>> On 04.08.2011 11:32, simendsjo wrote:
>>> I would like to use a template mixin to add some fields to a struct, but
>>> I'd also like the template to add additional invariant checks without
>>> having to remember to add this for all struct/classes that mixes in this
>>> code.
>>>
>>> class C {
>>> int a;
>>> }
>>>
>>> mixin template EmbedC() {
>>> C _c;
>>>
>>> // oops.. more than one invariant
>>> invariant() {
>>> assert(_c);
>>> }
>>>
>>> void close() {
>>> _c = null;
>>> }
>>> }
>>>
>>> struct S {
>>> int i = 10;
>>> invariant() {
>>> assert(i>= 10);
>>> }
>>>
>>> mixin EmbedC!();
>>> }
>>>
>>> void main() {
>>> S s;
>>> s.close();
>>> s._c.a = 10; // access violation, but I want assertion from invariant
>>> }
>>
>> Should I create a feature request to support more than one invariant?
>> It's pretty obvious that it works as designed given the nice error
>> message though..
>
> Feel free to create a feature request on it. It may even get the language
> changed. However, having more than one invariant complicates things a bit,
> since right now, it's easy for the runtime to just call the one invariant. If
> you had multiple invariants, they would have to be merged somehow by the
> compiler. It's completely doable, but it definitely complicates things, which
> is probably why it doesn't work that way now.
>
> If you wanted to be able to do it now though, I'd suggest that the mixins not
> have invariants but that they have functions which you then have to call in
> the invariant of the type that they're being mixed into. It's more error
> prone, since you could forget to call one of those functions in the invariant,
> but it allows you to have invariants of a sort in the mixed in code and still
> only have one invariant for the type.
>
> - Jonathan M Davis

http://d.puremagic.com/issues/show_bug.cgi?id=6453