Unfortunately a rather massive change. Gets rid of GncSqlColumnTypeHandler
and its hideous run-time registry by making the handler functions members
of GncSqlColumnTableEntry. GncSqlColumnTableEntry becomes an abstract super-
class with a templatized subclass GncSqlColumnTableEntryImpl whose template
parameter is the GncSqlObjectType that's also the GncSqlColumnTableEntryImpl's
m_col_type member. That member is a candidate for future removal as it's not
necessary. An explicit specialization is provided for each GncSqlObjectType and
that's a candidate for future code-shrinkage as there's still a fair amount
of duplication in spite of adding several helper template functions to absorb
most of the meat. The abstract GncSqlColumnTableEntry class is motivated by
the need to have an iterable container which in turn requires runtime
polymorphism. This can probably be replaced with a variadic-template
pseudo-container.
The two major benefits of this change are that it gets rid of one more set
of macro functions and provides vtable dispatch that has only one level
of indirection instead of the cumbersome string-hash-table-lookup to
find the handler followed by invoking the handler's function.
The two header files are removed because they exposed only the initialization
function which in turn only registered the respective GncSqlColumnTypeHandler.
