#include directive

#include directive physically includes the contents of a specified file into the current file.

Syntax:

#include "filename"

or

#include <filename>

When used in its first form, the referenced file is searched in the same folder as the current file.

For example, let us have following two files:

file1.h

struct A
{
    // …
};

And file2.h:

#include "file1.h"

struct B
{
    A a;
};

If there was no #include directive in file2.h, you would not be able to add this file to a Structure Library, as it uses an undefined type A. But preprocessor, which runs on the file before it is compiled, transforms the file into the following:

struct A
{
    // …
};

struct B
{
    A a;
};

That is, it physically inserts the contents of file1.h into file2.h, thus, making file2.h compilable. See also the #pragma once directive.

Using Absolute and Relative Paths

Both syntax forms, form 1 and form 2 allow you to specify absolute or relative paths, for example:

// will use an absolute path
#include "c:\Projects\definitions.h"

// includes "definitions.h" file, located in "inc" sibling
#include "..\inc\definitions.h"

// includes "definitions.h" file, located in "lib" subdirectory 
// of one of standard include paths.
#include <lib\definitions.h>