C alternative tokens explained
C alternative tokens refer to a set of alternative spellings of common operators in the C programming language. They are implemented as a group of macro constants in the C standard library in the iso646.h header. The tokens were created by Bjarne Stroustrup for the pre-standard C++ language[1] and were added to the C standard in a 1995 amendment to the C90 standard via library to avoid the breakage of existing code.
The alternative tokens allow programmers to use C language bitwise and logical operators which could otherwise be hard to type on some international and non-QWERTY keyboards. The name of the header file they are implemented in refers to the ISO/IEC 646 standard, a 7-bit character set with a number of regional variations, some of which have accented characters in place of the punctuation marks used by C operators.
The macros
The iso646.h header defines the following 11 macros as stated below:[2]
| Macro | Defined as |
|---|
and | && |
and_eq | &= |
bitand | & |
bitor | <nowiki>|</nowiki>|-| compl || ~|-| not || <nowiki>!</nowiki> |
not_eq | <nowiki>!=</nowiki> |
or | <nowiki>||</nowiki> |
or_eq | <nowiki>|</nowiki>=|-| xor || ^|-| xor_eq || ^=|}C++
The above-mentioned identifiers are operator keywords in the ISO C++ programming language and do not require the inclusion of a header file.[3] For consistency, the C++98 standard provided both <iso646.h> and a corresponding <ciso646>. However they both had no effect, being empty.[4] [5] Some compilers, such as Microsoft Visual C++ have, at least in the past, required the header to be included in order to use these identifiers unless a compiler flag is set.[6] [7] The header <ciso646> was deprecated in C++17, and removed in C++20,[8] while <iso646.h> was retained for compatibility with C.[9]
See also
- Digraphs and trigraphs in C
External links
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Notes and References
- Web site: CppCon 2017: Panel "Grill the Committee" . . https://ghostarchive.org/varchive/youtube/20211212/JYG5LFHkUuE. 2021-12-12 . live.
- Web site: Rationale for International Standard - Programming Languages - C . 5.10 . April 2003 . 2010-10-17 . live . https://web.archive.org/web/20160606072228/http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf . 2016-06-06.
- Book: Design and Evolution of C++ . Bjarne . Stroustrup . Bjarne Stroustrup . . 1st . 1994-03-29 . 0-201-54330-3.
- Book: Lischner, Ray . 0-596-00298-X . C++ in a Nutshell . 384 . . 2003 .
- Book: Peter. Van Weert. Marc. Gregoire. 2019. C++17 Standard Library Quick Reference. Apress. 9781484249239. 277.
- Book: Paul. Deitel. Harvey M.. Deitel. C++11 for Programmers. 2013. 9780133439878. Pearson Education. 928.
- Web site: 2021-11-23. Logical AND Operator:
&&. live. 2021-12-27. Microsoft Docs. https://web.archive.org/web/20211228013621/https://docs.microsoft.com/en-us/cpp/cpp/logical-and-operator-amp-amp?view=msvc-170 . 2021-12-28 .
- Web site: . 2021-12-07. Tyler. Whitney. Microsoft Learn.
- Thomas. Köppe. 2021-06-11. ISO/IEC JTC1 SC22 WG21. Clarifying the status of the "C headers". C++ Standards Committee Papers. live. 2024-01-11. https://web.archive.org/web/20240111125009/https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2340r1.html.
