Alternative operator representations

< cpp‎ | language

C++ (and 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), C++ defines two kinds of alternatives: additional keywords that correspond to the operators that use these characters and special combinations of two or three ISO 646 compatible characters that are interpreted as if they were a single non-ISO 646 character.


[edit] Alternative tokens

There are alternative spellings for several operators defined as tokens with special meaning in the C++ standard.

Primary Alternative
&& and
&= and_eq
& bitand
| bitor
~ compl
! not
!= not_eq
|| or
|= or_eq
^ xor
^= xor_eq

[edit] Compatibility with C

The same words are defined in the C programming language in the include file <iso646.h> as macros. Because in C++ these are built into the language, the C++ version of <iso646.h>, as well as <ciso646>, does not define anything.

[edit] Digraphs and trigraphs

The following combinations of two and three characters (digraphs and trigraphs) are valid substitutions for their respective primary characters:

Primary Digraph Trigraph(until C++17)
{ <% ??<
} %> ??>
[ <: ??(
] :> ??)
# %: ??=
\ ??/
^ ??'
| ??!
~ ??-

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 "Enter date ??/??/??" is parsed as "Enter date \\??".

(until C++17)

When the parser meets the charater sequence <:: and the subsequent character is neither : nor >, the < is treated as a preprocessor token by itself and not as the first character of the alternative token <:. Thus std::vector<::std::string> won't be wrongly treated as std::vector[:std::string>.

(since C++11)

[edit] Keywords

and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq

[edit] Example

The following example demonstrates the use of several alternative tokens.

%:include <iostream>
int main(int argc, char *argv<::>) 
    if (argc > 1 and argv<:1:> not_eq NULL) <%
        std::cout << "Hello, " << argv<:1:> << '\n';

[edit] See also

C documentation for Alternative operator representations