C source code may be written in any non-ASCII 7-bit character set that includes the ISO 646:1983 invariant character set. However, several C operators and punctuators require characters that are outside of the ISO 646 codeset: {, }, [, ], #, \, ^, |, ~. To be able to use character encodings where some or all of these symbols do not exist (such as the German DIN 66003), there are two possibilities: alternative spellings of operators that use these characters or special combinations of two or three ISO 646 compatible characters that are interpreted as if they were a single non-ISO 646 character.
Alternative spellings
There are alternative spellings for several operators defined in <iso646.h>.
| Defined in header <iso646.h> | |
|---|---|
| Primary | Alternative |
&& | and (macro constant) |
&= | and_eq (macro constant) |
& | bitand (macro constant) |
| | bitor (macro constant) |
~ | compl (macro constant) |
! | not (macro constant) |
!= | not_eq (macro constant) |
|| | or (macro constant) |
|= | or_eq (macro constant) |
^ | xor (macro constant) |
^= | xor_eq (macro constant) |
Digraphs and trigraphs
The following combinations of two and three characters (digraphs(C95) and trigraphs(C89)) are valid substitutions for their respective primary characters:
| Primary | Digraph | Trigraph |
|---|---|---|
{ | <% | ??< |
| } | %> | ??> |
[ | <: | ??( |
] | :> | ??) |
# | %: | ??= |
\ | ??/ | |
^ | ??' | |
| | ??! | |
~ | ??- |
Note that trigraphs (but not digraphs) are parsed before comments and string literals are recognized, so a comment such as // Will the next line be executed?????/ will effectively comment out the following line, and the string literal such as "What's going on??!" is parsed as "What's going on|".
Example
The following example demonstrates alternative operator spellings from the <iso646.h> header as well as use of digraphs and trigraphs.
The space character in the first command-line argument, argv[1], requires the quotation marks: ", World!".
1 2 3 4 5 6 7 8 9 10 11 12 13 | %:include <stdlib.h>%:include <stdio.h>%:include <iso646.h> int main(int argc, char** argv)??< if (argc > 1 and argv<:1:> not_eq NULL) <% printf("Hello%s\n", argv<:1:>); %> return EXIT_SUCCESS;??> |
Possible output:
1 | Hello, World! |
C++ documentation for Alternative operator representations |
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