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Bic() also supports em(polymorphic semantic values). Polymorphic semantic
values were developed as an alternative to using unions. Traditional unions
can still be used, but are now considered somewhat `old school'. Polymorphic
semantic values were added to b() as the result of a suggestion originally
made by Dallas A. Clement in September 2007.
In this section a simple example program is developed illustrating the use of
polymorphic semantic values. The sources of the example can be retrieved from
the distribution's lurl(poly) directory.
One may wonder why a tt(union) is still used by b() as bf(C++) offers
inherently superior approaches to combine multiple types into one union
type. The bf(C++) way to do so is by defining a polymorphic base class and a
series of derived classes implementing the various exclusive data types. The
tt(union) approach is still supported by b(), mainly for historic reasons as
it is supported by bf(bison)(1) and bf(bison++); dropping the tt(union) would
needlessly impede backward compatibility.
The preferred alternative to a tt(union), however, is a polymorphic base
class. The example program (cf. lurl(poly)) uses a polymorphic semantic value
type, supporting either tt(int) or tt(std::string) semantic values. These
types are asociated with tags (resp. tt(INT) and tt(TEXT)) using the
tt(%polymorphic) directive, discussed next.
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