In the CeeLanguage you can guard against the contents of a header file being parsed more than once by using an IncludeGuard:

  #ifndef HEADER_H_
  #define HEADER_H_

  // The whole rest of the file goes here

  #endif // The last line in the file

If the preprocessor sees this file a second time it will ignore everything between the #ifndef and the #endif - ''i.e.'' the whole body of the file. 

Are you bothered by the file being read every time it's included?  Then try RedundantIncludeGuards.

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It seems IncludeGuard''''''s is enough. I think of RedundantIncludeGuards as an implementation detail that hurts maintainability and should be something dealt with by the compiler. 

There's ''should'' ... and then there's the RealWorld.

Most tests on modern compilers (such as KAI or MSVC) show the difference in compilation time between normal IncludeGuard''''''s and RedundantIncludeGuards to be any where from non-existent to milliseconds. Are RedundantIncludeGuards worth the time saved? Not for me, but do your own experiments and decide which is more important to you - a OnceAndOnlyOnce IncludeGuard or the slightly faster compile times of RedundantIncludeGuards? Also don't forget there are multiple ways to skin a cat - using PrecompiledHeader''''''s or creating smaller lexical units, for example, are preferable to RedundantIncludeGuards.

''Modern'' is not necessarily wide-spread (remember, the RealWorld). Also, what size had the "most tests" systems. Pre-compiled headers introduce problems of their own, at least if they are implemented like in MSVC.

''RedundantIncludeGuards may be useless, but IncludeGuard''''''s are important to prevent classes, typedefs, macros, etc. from being declared multiple times, which can result in compile failure.''

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Am I the only person that wonders if recurring includes is a symptom of a system structure that needs fixing? (see LargeScaleCppSoftwareDesign ) -- SvenDowideit

Actually, while I don't agree, I think it was Lakos in LargeScaleCppSoftwareDesign that recommended the use of RedundantIncludeGuards.

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