REUSABLE COMPONENTS in Software

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A class developed and coded into separate files is a software component that can be used again and again in a number of different programs A reusable component saves effort because it does not need to be redesigned, recoded, and retested for every application A reusable component is also likely to be more reliable than a component that is used only once It is likely to be more reliable for two reasons First, you can afford to spend more time and effort on a component if it will be used many times Second, if the component is used again and again, it is tested again and again Every use of a software component is a test of that component Using a software component many times in a variety of contexts is one of the best ways to discover any remaining bugs in the software
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s USING #ifndef
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We have given you a method for placing a program in three (or more) les: two for the interface and implementation of each class and one for the application part of the program A program can be kept in more than three les For example, a program might use several classes, and each class might be kept in a separate pair of les Suppose you have a program spread across a number of les and that more than one le has an include directive for a class interface le such as the following:
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#include "dtimeh"
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Under these circumstances you can have les that include other les, and these other les may in turn include yet other les This can easily lead to a situation in which a le, in effect, contains the de nitions in dtimeh more than once C++ does not allow you to de ne a class more than once, even if the repeated de nitions are identical Moreover, if you are using the same header le in many different projects, it becomes close to impossible to keep track of whether you included the class de nition more than once To avoid this problem, C++ provides a way of marking a section of code to say if you have already included this stuff once before, do not include it again The way this is done is quite intuitive, although the notation may look a bit weird until you get used to it We will go through an example, explaining the details as we go The following directive de nes DTIME_H:
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#define DTIME_H
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What this means is that the compiler s preprocessor puts DTIME_H on a list to indicate that DTIME_H has been seen De ned is perhaps not the best word for this, since DTIME_H
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is not de ned to mean anything but merely put on a list The important point is that you can use another directive to test whether DTIME_H has been de ned and so test whether a section of code has already been processed You can use any (nonkeyword) identi er in place of DTIME_H, but you will see that there are standard conventions for which identi er you should use The following directive tests to see whether DTIME_H has been de ned:
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#ifndef DTIME_H
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If DTIME_H has already been de ned, then everything between this directive and the rst occurrence of the following directive is skipped:
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An equivalent way to state this, which may clarify the way the directives are spelled, is the following: If DTIME_H is not de ned, then the compiler processes everything up to the next #endif The not is why there is an n in #ifndef (This may lead you to wonder whether there is a #ifdef directive as well as a #ifndef directive There is, and it has the obvious meaning, but we will have no occasion to use #ifdef) Now consider the following code:
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