Include guards

From Cppwiki

Oh no! My code doesn't work! I get an error about multiple class definitions! Here's what my header, myclass.h, looks like:

// Here is my class, which does things.
class MyClass {
  void DoThings();
};

The problem occurs when MyClass is defined twice in a single translation unit, which generally happens when myclass.h is included twice. You don't necessarily need #include "myclass.h" twice in a single source file for this to occur - imagine a case where main.cpp includes a.h and b.h, both of which include myclass.h.

The solution, luckily, is easy.

#ifndef MYPROJECT_MYCLASS
#define MYPROJECT_MYCLASS
 
// Here is my class, which does things.
class MyClass {
  void DoThings();
};
 
#endif

Every time it includes the file, it checks to see if MYPROJECT_MYCLASS is defined. If it isn't, it defines MYPROJECT_MYCLASS - thus ensuring the body won't be included a second time - and then includes the body. If it is, it just skips the rest of the file (down to the #endif).

MYPROJECT_MYCLASS must be unique, otherwise you'll have bizarre issues with files not being included properly. How you guarantee it's unique is up to you - some people include their name, some people include a file path, some people include a timestamp or a random number. (E.g. AUTHOR_YYYYMMDD_HHMM contains some possibly useful information, isn't too long, and - most importantly - has a great deal of permanent global uniqueness.) Often, people start it with an underscore - however, this is incorrect and shouldn't be done, as those are reserved names.

#pragma once

"#pragma once" or a similar mechanism, when offered by a compiler, provides the following minor advantages to include guards:

• Reduces the possibility of various errors (e.g. #ifdef instead of #ifndef, forgetting the closing #endif, dependance of the guard name on the header name, repeating the same unique identifier twice)
• Does not pollute the global namespace with a macro
• Is slightly more likely to trigger a mechanism within the compiler which will speed up compilation by skipping over the header if it is included s second time
• Is slightly faster to type

It also has the following disadvantages:

• Is not standard C++
• Is not guaranteed to exist on all compilers
• Is not nearly as well-known or as common as include guards

The channel ops have mixed opinions regarding the use of "#pragma once".