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Below the way Bisonc++ may use a polymorphic semantic value is described. The
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described method is a direct result of a suggestion initially brought forward
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by Dallas A. Clement in September 2007.
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Bisonc++ may use polymorphic semantic values. How this is realized is covered
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in this section. The described method is a direct result of a suggestion
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initially brought forward by Dallas A. Clement in September 2007.
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One may wonder why a tt(union) is still used by Bisonc++ as bf(C++) offers
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One may wonder why tt(union)s are still used by Bisonc++, as bf(C++) offers
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inherently superior constructs to combine multiple types into one type. The
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bf(C++) way to combine types into one type is by defining a polymorphic base
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class and a series of derived classes implementing the alternative data
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types. Bisonc++ supports the tt(union) approach (and the unrestricted unions
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with C++0x) for various (e.g., backward compatibility)
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reasons. bi(Bison) and bi(bison++) both support the tt(%union) directive.
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support by C++11 for unrestricted unions may result in tt(union)s being
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re-evaluated, but bf(C++) traditionally offers a good alternative for the use
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of tt(union)s: a polymorphic base class and a series of derived classes
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implementing the alternative data types.
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An alternative to using a tt(union) is using a polymorphic base class. Such a
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class is developed below (the class tt(Base)). As it is a polymorphic
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base class it has the following characteristics:
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Below, a polymorphic base class (the class tt(Base)) is developed. As it is a
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polymorphic base class it has the following characteristics:
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it() Its destructor is virtual (and has a default implementation);
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it() Objects of the derived classes may be obtained from a
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yo/concrete/polymorphic.yo
