IMPLEMENT macros for defining wrapper functions for "hash" and "cmp" callbacks
that are specific to the underlying item type in a hash-table. This prevents
function pointer casting altogether, and also provides some type-safety
because the macro does per-variable casting from the (void *) type used in
LHASH itself to the type declared in the macro - and if that doesn't match the
prototype expected by the "hash" or "cmp" function then a compiler error will
result.
NB: IMPLEMENT macros are not required unless predeclared forms are required
(either in a header file, or further up in a C file than the implementation
needs to be). The DECLARE macros must occur after the type-specific hash/cmp
callbacks are declared. Also, the IMPLEMENT and DECLARE macros are such that
they can be prefixed with "static" if desired and a trailing semi-colon should
be appended (making it look more like a regular declaration and easier on
auto-formatting text-editors too).
Now that these macros are defined, I will next be commiting changes to a
number of places in the library where the casting was doing bad things. After
that, the final step will be to make the analogous changes for the lh_doall
and lh_doall_arg functions (more specifically, their callback parameters).
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