#include directive (C/C++)

Tells the preprocessor to include the contents of a specified file at the point where the directive appears.

Syntax

#include " path-spec "
#include < path-spec >

Remarks

You can organize constant and macro definitions into include files (also known as header files) and then use #include directives to add them to any source file. Include files are also useful for incorporating declarations of external variables and complex data types. The types may be defined and named only once in an include file created for that purpose.

The path-spec is a file name that may optionally be preceded by a directory specification. The file name must name an existing file. The syntax of the path-spec depends on the operating system on which the program is compiled.

For information about how to reference assemblies in a C++ application that's compiled by using /clr, see #using directive.

Both syntax forms cause the #include directive to be replaced by the entire contents of the specified file. The difference between the two forms is the order of the paths that the preprocessor searches when the path is incompletely specified. The following table shows the difference between the two syntax forms.

The preprocessor stops searching as soon as it finds a file that has the given name. If you enclose a complete, unambiguous path specification for the include file between double quotation marks (" "), the preprocessor searches only that path specification and ignores the standard directories.

If the file name that's enclosed in double quotation marks is an incomplete path specification, the preprocessor first searches the parent file's directory. A parent file is the file that contains the #include directive. For example, if you include a file named file2 in a file named file1, file1 is the parent file.

Include files can be nested: An #include directive can appear in a file that's named by another #include directive. For example, file2 could include file3. In this case, file1 would still be the parent of file2, but it would be the grandparent of file3.

When include files are nested and when compiling occurs on the command line, directory searching begins in the directory of the parent file. Then it proceeds through the directories of any grandparent files. That is, searching begins relative to the directory that contains the source that's currently being processed. If the file isn't found, the search moves to directories that are specified by the /I (Additional include directories) compiler option. Finally, the directories that are specified by the INCLUDE environment variable are searched.

Within the Visual Studio development environment, the INCLUDE environment variable is ignored. The values specified in the project properties for include directories are used instead. For more information about how to set the include directories in Visual Studio, see Include Directories and Additional Include Directories.

This example shows file inclusion by using angle brackets:

#include <stdio.h>

The example adds the contents of the file named stdio.h to the source program. The angle brackets cause the preprocessor to search the directories that are specified by the INCLUDE environment variable for stdio.h, after it searches directories that are specified by the /I compiler option.

The next example shows file inclusion by using the quoted form:

#include "defs.h"

The example adds the contents of the file that's specified by defs.h to the source program. The quotation marks mean that the preprocessor first searches the directory that contains the parent source file.

Nesting of include files can continue up to 10 levels. When processing of the nested #include is finished, the preprocessor continues to insert the enclosing parent include file into the original source file.

Microsoft-specific

To locate the source files to include, the preprocessor first searches the directories specified by the /I compiler option. If the /I option isn't present, or if it fails, the preprocessor uses the INCLUDE environment variable to find any include files within angle brackets. The INCLUDE environment variable and /I compiler option can contain multiple paths, separated by semicolons (;). If more than one directory appears as part of the /I option or within the INCLUDE environment variable, the preprocessor searches them in the order in which they appear.

For example, the command

CL /ID:\msvc\include myprog.c

causes the preprocessor to search the directory D:\msvc\include\ for include files such as stdio.h. The commands

SET INCLUDE=D:\msvc\include
CL myprog.c

have the same effect. If both sets of searches fail, a fatal compiler error is generated.

If the file name is fully specified for an include file that has a path that includes a colon (for example, F:\MSVC\SPECIAL\INCL\TEST.H), the preprocessor follows the path.

For include files that are specified as #include "path-spec", directory search begins in the directory of the parent file and then proceeds through the directories of any grandparent files. That is, the search begins relative to the directory that contains the source file that's being processed. If there's no grandparent file and the file still isn't found, the search continues as if the file name were enclosed in angle brackets.

END Microsoft-specific

See also

Preprocessor directives
/I (Additional include directories)