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31 lines
1.7 KiB
Text
31 lines
1.7 KiB
Text
In the previous chapters we provided examples of classes where each object
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had its own set of data members data. Each of the class's
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member functions could access any member of any object of its class.
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In some situations it may be desirable to define
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emi(common data fields), that may be accessed by em(all) objects of the
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class. For example, the name of the startup directory, used by a program that
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recursively scans the directory tree of a disk. A second example is a variable
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that indicates whether some specific initialization has occurred. In that case
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the object that was constructed first would perform the initialization and
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would set the flag to `done'.
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Such situations are also encountered in bf(C), where several functions need to
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access the same variable. A common solution in bf(C) is to define all these
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functions in one source file and to define the variable ti(static): the
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variable name is invisible outside the scope of the source file. This approach
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is quite valid, but violates our philosophy of using only one function per
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source file. Another bf(C)-solution is to give the variable in question an
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unusual name, e.g., tt(_6uldv8), hoping that other program parts won't use
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this name by accident. Neither the first, nor the second legacy bf(C) solution
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is elegant.
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bf(C++) solves the problem by defining ti(static members): data and functions,
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common to all objects of a class and (when defined in the private section)
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inaccessible outside of the class. These static members are this chapter's
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topic.
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Static members cannot be defined as hi(virtual: vs static) virtual
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functions. A virtual member function is an ordinary member in that it has a
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ti(this) pointer. As static member functions have no tt(this) pointer, they
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cannot be declared virtual.
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