Question 10.7

Is it acceptable for one header file to #include another?

It's a question of style, and thus receives considerable debate. Many people believe that ``nested #include files'' are to be avoided: the prestigious Indian Hill Style Guide (see question 17.9) disparages them; they can make it harder to find relevant definitions; they can lead to multiple-definition errors if a file is #included twice; and they make manual Makefile maintenance very difficult. On the other hand, they make it possible to use header files in a modular way (a header file can #include what it needs itself, rather than requiring each #includer to do so); a tool like grep (or a tags file) makes it easy to find definitions no matter where they are; a popular trick along the lines of:

	#ifndef HFILENAME_USED
	#define HFILENAME_USED
	...header file contents...
	#endif

(where a different bracketing macro name is used for each header file) makes a header file ``idempotent'' so that it can safely be #included multiple times; and automated Makefile maintenance tools (which are a virtual necessity in large projects anyway; see question 18.1) handle dependency generation in the face of nested #include files easily. See also question 17.10.

References: Rationale Sec. 4.1.2

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