Include guard

Include guard
http://www.bobarcher.org/software/include/index.html

節錄翻譯:

為避免重複include，須加上include guard。
以下為internal include guard

File "myheader.hpp"

#ifndef MYHEADER_HPP

#define MYHEADER_HPP

// Contents of include file go here

#endif

但上面的方法用在非常大的project時，會造成compile的時間增加。
This point is made by John Lakos in his book "Large Scale C++ Software Design", and is used as an argument to support the following idiom:

File "myheader2.hpp"

#ifndef MYHEADER_HPP

#include "myheader.hpp"

#endif

// Rest of header file goes here

...

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http://en.wikipedia.org/wiki/Include_guard
http://ccckmit.wikidot.com/cp:includeguard