Aninclude directive instructs atext file processor to replace the directive text with the content of a specified file.
The act of including may be logical in nature. The processor may simply process the include file content at the location of the directive without creating a combined file.
Different processors may use different syntax. TheC preprocessor (used withC,C++ and in other contexts) defines an include directive as a line that starts#include and is followed by a file specification.COBOL defines an include directive indicated bycopy in order to include acopybook.
Generally, for C/C++ the include directive is used to include aheader file, but can include any file. Although relatively uncommon, it is sometimes used to include abody file such as a .c file.
The include directive can supportencapsulation andreuse. Different parts of a system can be segregated into logical groupings yet rely on one another via file inclusion. C and C++ are designed to leverage include while also optimizing build time by allowingdeclaration separate fromimplementation. The included information can be minimized to only declarations.
As many consider that including file content has significant drawbacks, newer languages have been designed without an include directive. Languages such asJava andC# support modularization via animport concept that allows a module to use the assets of another module at a conceptual level; not by including text.
Both C and C++ are typically used with the C preprocessor that replaces a#include directive line with the content of the file specified. A file path is either enclosed in double quotes (i.e. "xyz.h") or angle brackets (i.e. <xyz.h>).[1] Some preprocessors locate the include file differently based on the enclosing delimiters; treating a path in double-quotes as relative to the including file and a path in angle brackets as located in one of the directories of the configured system search path.[2]
Example include statements:
// include the C standard header 'stdio.h'; probably is a file with that name#include<stdio.h>// include the C++ standard header 'vector'; may or may not be a file#include<vector>// include a custom header file named 'user_defined.h'#include"user_defined.h"
The include directive allows for the development of codelibraries that:
Given two C source files. One defines a functionadd() and another uses the function. Without using an include directive, the consuming file can declare the function locally as afunction prototype:
intadd(int,int);inttriple(intx){returnadd(x,add(x,x));}
One drawback of this approach is that the function prototype must be present in each file that callsadd(). Another drawback is that if the signature changes, then each consuming file needs to be updated. Putting the prototype in a single, separate file avoids these issues. If the prototype is moved to a header fileadd.h, the using source file becomes:
#include"add.h"inttriple(intx){returnadd(x,add(x,x));}
In C and C++, aheader file is asource code file that allowsprogrammers to separate elements of acodebase – often into reusable, logically-related groupings.
A header filedeclares programming elements such asfunctions,classes,variables, and preprocessormacros. A header file allows the programmer to use programming elements in multiple body files based on the common declaration in the header file. Declarations in a header file allow body files to use implementations without including the implementation code directly. The header keeps the interface separate from theimplementation.[3]
Compilation errors may occur if multiple header files include the same file. One solution is to avoid including files in header files – possibly requiring excessive include directives in body files. Another solution is to use aninclude guard in each header file.[4]
TheC standard library is declared as a collection of header files. TheC++ standard library is similar, but the declarations may be provided by the compiler without reading an actual file.
C standard header files are named with a.hfile name extension, as in#include <stdio.h>. Typically, custom C header files have the same extension. Custom C++ header files tend to have more variety of extensions, including.hpp,.h++ and.hh.
A C++ standard library name in angle brackets (i.e. <vector>) results in declarations being included but may not be from a file.[5]
SinceC++20, C++ supports import semantics via theheader unit, that is, separate translation units synthesized from a header.[6] They are meant to be used alongsidemodules. The syntax used in that case is:
exportoptional import header-name;
Example:
import<stdio.h>;// supporting this is optionalimport<vector>;// supporting this is mandated by the standardexportimport"user_defined.h";
Header units are provided for all the C++ standard library headers.[7]
COBOL (and alsoRPG IV) allows programmers to copy copybooks into the source of the program – which is similar to including but allows for replacing text. The COBOL keyword for inclusion isCOPY, and replacement is done using theREPLACING ... BY ... clause. An include directive has been present in COBOL since COBOL 60, but changed from the originalINCLUDE[8] toCOPY by 1968.[9]
Fortran does not require header filesper se. However, Fortran 90 and later have two related features:include statements and modules. The former can be used to share a common file containing procedure interfaces, much like a C header, although the specification of an interface is not required for all varieties of Fortran procedures. This approach is not commonly used; instead, procedures are generally grouped into modules that can then be referenced with ause statement within other regions of code. For modules, header-type interface information is automatically generated by the compiler and typically put into separate module files, although some compilers have placed this information directly into object files. The language specification itself does not mandate the creation of any extra files, even though module procedure interfaces are almost universally propagated in this manner.
MostPascal compilers support the$i or$include compiler directive, in which the$i or$include directive immediately follows the start of a comment block in the form of
{$ifilename.pas}(*$Ifilename.inc*){$includefilename.inc}(*INCLUDEfilename.pas*)Where the$i or$include directive is notcase sensitive, andfilename.pas orfilename.inc is the name of the file to be included. (It has been common practice to name Pascal's include files with theextension.inc, but this is not required.) Some compilers, to prevent unlimited recursion, limit invoking an include file to a certain number, prohibit invoking itself or any currently open file, or are limited to a maximum of one include file at a time, e.g. an include file cannot include itself or another file. However, the program that includes other files can include several, just one at a time.
InPHP, theinclude directive causes another PHP file to be included and evaluated.[10] Similar commands arerequire, which upon failure to include will produce afatal exception and halt the script,[11] andinclude_once andrequire_once, which prevent a file from being included or required again if it has already been included or required, avoiding the C's double inclusion problem.
Other notable languages with an include directive:
include ... (Fortran,MASM)<!--#include ... --> (HTMLSSI)var ... = require("...") (JavaScript withCommonJS)<%@ include ... %> (JSP){$I ...} (UCSD Pascal,Turbo Pascal)%include ... (PL/I)/COPYQCPYLESRC,QBC (RPG IV – first argument is the filename, second argument is the copybook)local ... = require("...") (Lua)import ...; (D)Modern languages (e.g.Haskell andJava) tend to avoid the include directive construct, preferringmodules and import/export semantics. Some of these languages (such as Java andC#) do not use forward declarations and, instead, identifiers are recognized automatically from source files and read directly fromdynamic library symbols (typically referenced withimport orusing directives).
< and> delimited sequences in header names necessarily valid source file names.