Having studied the mechanisms, the next step is to find proper ways,
recipes as it were, to use these mechanisms.
What we need, in the terminology of
In this section, we will look at the concrete class idiom for C++, which states the ingredients that every class must have to behave as if it were a built-in type. Other idioms, in particular an improved version of the handle/body or envelope/letter idiom that may be used to separate interface from implementation, will be treated in the next section.
Abstract data types must be indistinguishable from built-in types
To verify whether a concrete data type meets the requirements imposed by the specification of the abstract data type is quite straightforward, although not always easy. However, the task of verifying whether a concrete data type is optimally implemented is rather less well defined. To arrive at an optimal implementation may involve a lot of skill and ingenuity, and in general it is hard to decide whether the right choices have been made. Establishing trade-offs and making choices, for better or worse, is a matter of experience, and crucially depends upon the skill in handling the tools and mechanisms available.
When defining concrete data types, the list of requirements defining the canonical class idiom given in slide 2-canonical may be used as a check list to determine whether all the necessary features of a class have been defined. Ultimately, the programmer should strive to realize abstract data types in such a way that their behavior is in some sense indistinguishable from the behavior of the built-in data types. Since this may involve a lot of work, this need not be a primary aim in the first stages of a software development project. But for class libraries to work properly, it is simply essential.
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12/29/99 |
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