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POGNU Objective-C runtime trickery
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copied!<p>Can I, in the GNU Objective-C runtime, attach semi-arbitrary pieces of data to instance variables?</p> <h3>Challenge:</h3> <p>I'm currently working on a kind of Cocoa workalike for Linux, as a sort of pet project. (Please, let's not get sidetracked by all the "use GNUStep" stuff. I know about it, but it doesn't suit my needs. Moving on…) For this purpose I'm trying to cobble together a simple ORM system, reminiscent of DBIx::Class for Perl. The general idea is to make the declaration as simple (read: short) as possible, and if at all possible, without the need to provide <code>+(id)constantClassAttribute</code> methods for overriding.</p> <p>The general idea is to declare my result classes as follows:</p> <pre><code>@interface SomeTable : ORMResult { unsigned long long id; ORMResult *toOneRelation; ORMResultSet *toManyRelation; } @end </code></pre> <p>So far, so hoopy. I can now access these fields using <code>[ORMResult self]->ivars</code>, and do all manner of nasty stuff, like automagically generating accessors like <code>-[toManyRelation]</code> or <code>-[setToOneRelation]</code>. Piece of cake. Unfortunately, there are two pieces of information I cannot add using this setup; one is simple enough to solve, the other not so much:</p> <ol> <li><p>What is the actual result class?</p> <p>This is solved by subclassing <code>ORMResult</code> (like <code>SomeTable</code>), and plugging that in there, using runtime dynam(ag)ics to figure out it's to-ness (toMany, toOne).</p></li> <li><p>(And this is the tricky one!) Is the relationship nullable?</p> <p>This is less easily solved. My initial ideas were</p> <ol> <li><p>(ab)using protocols, like so:</p> <pre><code>@interface SomeTable : ORMResult { unsigned long long id; ORMResult <ORMNullable> *toOneRelation; } @end </code></pre> <p>This compiles, but unfortunately, when I try to use GDB to inspect the <code>ivars->ivar_list</code> entries I find that the protocol information isn't actually kept for the runtime to toy with. This makes, I suppose, some kind of twisted sense, as protocol declarations are mostly for the compiler.</p></li> <li><p>Abusing the protocol identifiers (<code>byref</code>, <code>bycopy</code> and friends, using defines:</p> <pre><code>@interface SomeTable : ORMResult { unsigned long long id; nullable OMRResult *toOneRelation; } @end </code></pre> <p>This has the rather obvious drawback of not actually working, as these specifiers apparently only work in protocol method declarations.</p></li> </ol></li> </ol> <p>The question, then, is how can this attachment of information to the ivars be pulled off in practice?</p> <p><strong>Note:</strong> As mentioned initially, I'm using the <em>GNU Objective-C runtime</em>, as supplied by GCC on Linux; and <em>not the one supplied by Apple!</em></p> <p><strong>Edit:</strong> Starpox! I forgot a central point: An alternative, of course, is to simply make <em>all</em> relations nullable. This I don't really want, but if no other alternative exists, I guess that's the path I'll end up going down.</p>

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