#include anywhere

Question

Is the #include meant to be used for headers only or is it simply a mechanical \"inject this code here\" that can be used anywhere in the code?

Answer 1

Compilers will not complain, but everyone who has to maintain the code will.

Answer 2

It is a mechanical inject the code here device. You can include a text file containing Goethe's Faust if you wish to. You can put it anywhere, even in the middle of a function (of course, #include needs a fresh line!).

However, it's strong convention to only use #include for header files. There may be reasons where I wouldn't object on it, for example pulling in machine-generated code or merging all translation units in a single file.

Answer 3

#include and other preprocessor directives like #define or #import, can appear anywhere in the source, but will only apply to the code after that inclusion. It is meant to include the referenced code into the source file that calls it. This MSDN page explains it quite well. http://msdn.microsoft.com/en-us/library/36k2cdd4(v=VS.71).aspx

Answer 4

Should work, it's processed by your preprocessor, your compiler won't even see it.

Answer 5

include is handled by the preprocessor and is a mechanism to inject code. There are no restrictions on the file being included or where this #include is placed in your code (thought it should be in its own line). As long as the file specified can be found by the preprocessor it will import its contents into the current file.

Conventionally you do this for header files. I've seen this being done with cpp files during template instantiation (with proper #ifdef so you don't include it multiple times causing multiple symbol definition error).

If you have a long constant, you can do this for other file types as well. (Though there are better ways of handling long string constants)

Answer 6

Not only does it work anywhere, but it can lead to some interesting techniques. Here's an example that generates an enumeration and a corresponding string table that are guaranteed to be in sync.

Animals.h:

ANIMAL(Anteater)
ANIMAL(Baboon)
...
ANIMAL(Zebra)

AnimalLibrary.h:

#define ANIMAL(name) name,

enum Animals {
#include "Animals.h"
        AnimalCount
    };

#undef ANIMAL

extern char * AnimalTable[AnimalCount];

AnimalLibrary.cpp:

#include "AnimalLibrary.h"

#define ANIMAL(name) #name,

char * AnimalTable[AnimalCount] = {
#include "Animals.h"
    };

main.cpp:

#include "AnimalLibrary.h"

int main()
{
    cout << AnimalTable[Baboon];
    return 0;
}

Be sure not to put the usual include guards in any file that will be included multiple times!

Gotta agree with William Pursell though that this technique will have people scratching their heads.