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CS253 Include Guards

Multiple Inclusion

• #include-ing a file twice can cause problems.
  Remember what #include does. It literally copies everything in the file into the current input stream, to be compiled along with the contents of the current file. If you #include a file twice, you compile it twice.
• If that file defines a class, you get an error saying that the class is already defined.

class Foo {
};

class Foo {  // 🦡
};

c.cc:4: error: redefinition of ‘class main()::Foo’

• Yeah, so? What sort of fool would #include something twice?

How it happens

main.cc:

    #include "foo.h"
    #include "bar.h"

foo.h:

    #include <string>

bar.h:

    #include <string>

Both foo.h and bar.h use strings, so they both #include <string>. However, we don’t want to have an error complaining that class string is redefined. How do we cure this?

The solution

string:

    #ifndef STRING_INCLUDED
    #define STRING_INCLUDED
    class string {
        …
    };
    #endif /* STRING_INCLUDED */

• The first time <string> was #included, the symbol STRING_INCLUDED was not defined. Therefore the rest of the compile-time #ifndef (if not defined) conditional compilation was processed.
• The preprocessor symbol STRING_INCLUDED was defined.
• The class string was defined.
• Next time, STRING_INCLUDED was defined, so the file had no effect. It was as if an occult hand had prevented the file from being processed a second time.

Non-standard solutions

• There exists a non-standard solution, #pragma once. Avoid it.
• It’s non-standard, and so doesn’t work everywhere.
• Include guards are standard, and so work everywhere.
• For current compilers, include guards are just as fast as #pragma once.