(C++) \#include guard

January 9, 2018 · View on GitHub

 

 

 

 

 

(C++) #include guard

 

An #include guard prevents a header file to be compiled multiple times (this will happen if a header file is #included by multiple header files, which will cause a redeclaration error).

 

An #include guard looks like the code below:

 

#ifndef MYHEADERFILE_H #define MYHEADERFILE_H //Your header file code, guaranteed to be compiled only once #endif

 

An implementation (.cpp) file does not have an #include guard, because they do not get #included, but 'Added to Project' instead.

 

Always write #include guards [1-5]. Always write internal #include guards [1-4]. Never write external #include guards [1,4]. Do write external #include guards [6]. Use a unique and predictable name for the #include guard [3].

 

 

 

 

 

References

 

  1. Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 24: 'Always write internal #include guards. Never write external #include guards'
  2. Jesse Liberty. Sams teach yourself C++ in 24 hours. ISBN:0-672-32224-2. Hour 21, chapter 'Inclusion and inclusion guards': 'It never hurts to use inclusion guards. Often they will save you hours of debugging time'. Also: hour 24, chapter 'include guards': 'All header files should use inclusion guards'
  3. John Lakos. Large-Scale C++ Software Design. 1996. ISBN: 0-201-63362-0. Chapter 2.4: 'Place a unique and predicatable (internal) include guard around the contents of each header file'
  4. Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 27: '#ifndef, #define and #endif will be used to prevent multiple inclusions of the same header file. Other techniques to prevent the multiple inclusions of header files will not be used.'
  5. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 15.5. Advice. page 444: '[7] Use include guards'
  6. John Lakos. Large-Scale C++ Software Design. 1996. ISBN: 0-201-63362-0. Section 2.5, page 85: 'Place a redundant (external) include guard around each preprocessor include directive in every header file'