For the purposes of this question, I am interested only in Standard-Compliant C++, not C or C++0x, and not any implementation-specific details.
Questions arise from time to time regarding the difference between #include ""
and #include <>
. The argument typically boils down to two differences:
#include <>
is for "headers" whereas #include ""
is for a "source file." Here is the relevant reference:ISO/IEC 14882:2003(E)
16.2 Source file inclusion [cpp.include]
1 A #include directive shall identify a header or source file that can be processed by the implementation.
2 A preprocessing directive of the form
searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined.# include < h-char-sequence > new-line
3 A preprocessing directive of the form
causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read# include "q-char-sequence" new-line
with the identical contained sequence (including > characters, if any) from the original directive.# include < h-char-sequence > new-line
(Emphasis in quote above is mine.) The implication of this difference seems to be that the Standard intends to differentiate between a 'header' and a 'source file', but nowhere does the document define either of these terms or the difference between them.
There are few other places where headers or source files are even mentioned. A few:
158) A header is not necessarily a source file, nor are the sequences delimited by in header names necessarily valid source file names (16.2).
Seems to imply a header may not reside in the filesystem, but it doesn't say that source files do, either.
2 Lexical conventions [lex]
1 The text of the program is kept in units called source files in this International Standard. A source file together with all the headers (17.4.1.2) and source files included (16.2) via the preprocessing directive
#include
, less any source lines skipped by any of the conditional inclusion (16.1) preprocessing directives, is called a translation unit. [Note: a C + + program need not all be translated at the same time. ]
This is the closest I could find to a definition, and it seems to imply that headers are not the "text of the program." But if you #include
a header, doesn't it become part of the text of the program? This is a bit misleading.
So what is a header? What is a source file?
So what's the difference between Header files and Source files? Basically, header files are #included and not compiled, whereas source files are compiled and not #included. You can try to side-step these conventions and make a file with a source extension behave like a header or vice-versa, but you shouldn't.
(1) A file that contains program instructions. See source code. (2) A file that contains original or essential data that is the starting point for a system of publishing or other processing.
C++ source files generally have the . cpp, . cxx or . cc extension suffixes. A C++ source file can include other files, known as header files, with the #include directive.
Header files are human-readable. Since they are in the form of source code.
My reading is that the standard headers, included by use of <>
angle brackets, need not be actual files on the filesystem; e.g. an implementation would be free to enable a set of "built-in" operations providing the functionality of iostream
when it sees #include <iostream>
.
On the other hand, "source files" included with #include "xxx.h"
are intended to be literal files residing on the filesystem, searched in some implementation-dependent manner.
Edit: to answer your specific question, I believe that "headers" are limited only to those #include
able facilities specified in the standard: iostream
, vector
and friends---or by the implementation as extensions to the standard. "Source files" would be any non-standard facilities (as .h
files, etc.) the programmer may write or use.
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