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CS253 Include Guards

Multiple Inclusion

#include-ing a file twice can cause problems.
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> is #included, the symbol STRING_INCLUDED is not defined. Therefore the rest of the compile-time #ifndef (if not defined) conditional compilation is processed.
The preprocessor symbol STRING_INCLUDED is defined.
The class string is defined.
Next time, STRING_INCLUDED is defined, so the file does nothing.

Non-standard solutions

There exists a non-standard solution, #pragma once.
It’s non-standard, and so doesn’t work everywhere.
Current compilers detect include guards, and so they are just as fast as #pragma once.
Include guards are standard, and so work everywhere.

CS253: Software Development with C++

Spring 2019

Include Guards

CS253 Include Guards

Multiple Inclusion

How it happens

The solution

Non-standard solutions