Matthew's Journal

<Diziet> You can use a union, but either that wastes memory by allocating enough memory for large representations when you have a small one, or it's a lie because it's not really a union, just the head of one.
<Diziet> So struct ByteArrayMatrix { GeneralMatrix general; actual stuff goes here; }
<Diziet> And then when you write bytematrix_test you cast the incoming GeneralMatrix* to a ByteArrayMatrix*.
<Diziet> The caller never allocates one. They just call GeneralMatrix *bytematrix_create(....)
<Diziet> Then they have GeneralMatrix *matrix which they can do matrix->vtable->test(matrix, ...) (via a macro if you want to do it lots).
<Diziet> s/which/with &/
<Diziet> bytematrix_create has a ByteArrayMatrix *bam= malloc(sizeof(*bam)) and returns &bam->general.
<Diziet> You should put a comment near GeneralMatrix general; in each SpecificKindOfMatrix saying not to move it from being the first member.
<Diziet> (C guarantees that the address of the first member is the same as the address of the whole struct, modulo type conversions.)
<Diziet> (If you need it to not be the first member, eg because something has already nabbed that trick, you can use offsetof but it's quite tedious.)
<ewx> if you have to use offsetof then hide the offsetofery behind a macro rather than typing it out lots of times
<Diziet> Oh, and of course you shouldn't call these things Matrix because they're not matrices.