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Reasons to Use Global Data
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Data purists sometimes argue that programmers should never use global data, but most programs use global data when the term is broadly construed. Data in a
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13. Unusual Data Types
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database is global data, as is data in configuration files such as the Windows registry. Named constants are global data, just not global variables. Used with discipline, global variables are useful in several situations:
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Preservation of global values Sometimes you have data that applies conceptually to your whole program. This might be a variable that reflects the state of a program for example, interactive vs. command-line mode, or normal vs. error-recovery mode. Or it might be information that s needed throughout a program for example, a data table that every routine in the program uses. Emulation of named constants Although C++, Java, Visual Basic, and most modern languages support named constants, some languages such as Python, Perl, Awk, and Unix shell script still don t. You can use global variables as substitutes for named constants when your language doesn t support them. For example, you can replace the literal values 1 and 0 with the global variables TRUE and FALSE set to 1 and 0, or replace 66 as the number of lines per page with LINES_PER_PAGE = 66. It s easier to change code later when this approach is used, and the code tends to be easier to read. This disciplined use of global data is a prime example of the distinction between programming in vs. programming into a language, which is discussed more in Section 34.4, Program Into Your Language, Not In It. Emulation of enumerated types You can also use global variables to emulate enumerated types in languages such as Python that don t support enumerated types directly. Streamlining use of extremely common data Sometimes you have so many references to a variable that it appears in the parameter list of every routine you write. Rather than including it in every parameter list, you can make it a global variable. In cases in which a variable seems to be accessed everywhere, however, it rarely is. Usually it s accessed by a limited set of routines you can package into a class with the data they work on. More on this later. Eliminating tramp data Sometimes you pass data to a routine or class merely so that it can be passed to another routine or class. For example, you might have an error-processing object that s used in each routine. When the routine in the middle of the call chain doesn t use the object, the object is called tramp data. Use of global variables can eliminate tramp data.
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For more details on named 6 constants, see Section 12.7, 7 Named Constants.
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5 CROSS-REFERENCE
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13. Unusual Data Types
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Use Global Data Only as a Last Resort
Before you resort to using global data, consider a few alternatives.
Begin by making each variable local and make variables global only as you need to Make all variables local to individual routines initially. If you find they re needed elsewhere, make them private or protected class variables before you go so far as to make them global. If you finally find that you have to make them global, do it, but only when you re sure you have to. If you start by making a variable global, you ll never make it local, whereas if you start by making it local, you might never need to make it global. Distinguish between global and class variables Some variables are truly global in that they are accessed throughout a whole program. Others are really class variables, used heavily only within a certain set of routines. It s OK to access a class variable any way you want to within the set of routines that use it heavily. If other routines need to use it, provide the variable s value by means of an access routine. Don t access class values directly as if they were global variables even if your programming language allows you to. This advice is tantamount to saying Modularize! Modularize! Modularize! Use access routines Creating access routines is the workhorse approach to getting around problems with global data. More on that in the next section.
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