In computer programming, digraphs and trigraphs are sequences of two and three characters, respectively, that appear in source code and, according to a programming language's specification, should be treated as if they were single characters.
Contents
Various reasons exist for using digraphs and trigraphs: keyboards may not have keys to cover the entire character set of the language, input of special characters may be difficult, text editors may reserve some characters for special use and so on. Trigraphs might also be used for some EBCDIC code pages that lack characters such as { and }.
History
The basic character set of the C programming language is a subset of the ASCII character set that includes nine characters which lie outside the ISO 646 invariant character set. The ANSI C committee invented trigraphs as a way of entering source code using keyboards that support any version of the ISO 646 character set.
Implementations
Trigraphs are not commonly encountered outside compiler test suites. Some compilers support an option to turn recognition of trigraphs off, or disable trigraphs by default and require an option to turn them on. Some can issue warnings when they encounter trigraphs in source files. Borland supplied a separate program, the trigraph preprocessor, to be used only when trigraph processing is desired (the rationale was to maximise speed of compilation).
Language support
Different systems define different sets of digraphs and trigraphs, as described below.
ALGOL
Early versions of ALGOL predated the standardised ASCII and EBCDIC character sets, and were typically implemented using a manufacturer-specific six-bit character code. A number of ALGOL operations either lacked codepoints in the available chacater set or were not supported by peripherals, leading to a number of substitutions including := for ← (assignment) and >= for ≥ (greater than or equal).
Pascal
Pascal programming language supports digraphs (., .), (* and *) for [, ], { and } respectively. Unlike all other cases mentioned here, (* and *) were and still are in wide use.
Vim
Vim text editor supports digraphs for actual entry of text characters, following RFC 1345.
GNU Screen
GNU Screen has a digraph command, bound to ^A ^V by default.
J
The J programming language is a descendant of APL using the ASCII character set rather than traditional APL symbols. To handle the fact that the printable range of ASCII is smaller than APL's specialised set of symbols, . (dot) and : (colon) characters are used to inflect ASCII symbols, effectively interpreting unigraphs, digraphs or rarely trigraphs as standalone "symbols".
Unlike the use of digraphs and trigraphs in C and C++, there are no single-character equivalents to these in J.
C
The C preprocessor replaces all occurrences of the following nine trigraph sequences by their single-character equivalents before any other processing.
A programmer may want to place two question marks together yet not have the compiler treat them as introducing a trigraph. The C grammar does not permit two consecutive ? tokens, so the only places in a C file where two question marks in a row may be used are in multi-character constants, string literals, and comments. This is particularly a problem for the classic Mac OS, where the constant '????' may be used as a file type or creator. To safely place two consecutive question marks within a string literal, the programmer can use string concatenation "...?""?..." or an escape sequence "...??...".
??? is not itself a trigraph sequence, but when followed by a character such as - it will be interpreted as ? + ??-, as in the example below which has 16 (5x3+1) ?s before the /.
The ??/ trigraph can be used to introduce an escaped newline for line splicing; this must be taken into account for correct and efficient handling of trigraphs within the preprocessor. It can also cause surprises, particularly within comments. For example:
which is a single logical comment line (used in C++ and C99), and
/??/ * A comment *??/ /which is a correctly formed block comment.
In 1994 a normative amendment to the C standard, included in C99, supplied digraphs as more readable alternatives to five of the trigraphs. They are listed in the table on the right.
Unlike trigraphs, digraphs are handled during tokenization, and any digraph must always represent a full token by itself, or compose the token %:%: replacing the preprocessor concatenation token ##. If a digraph sequence occurs inside another token, for example a quoted string, or a character constant, it will not be replaced.
C++
C++ (through C++14, see below) behaves like C, including the C99 additions, but with additional tokens listed in the table on the right.
As a note, %:%: is treated as a single token, rather than two occurrences of %:.
The C++ Standard makes this comment with regards to the term "digraph":
The term "digraph" (token consisting of two characters) is not perfectly descriptive, since one of the alternative preprocessing-tokens is %:%: and of course several primary tokens contain two characters. Nonetheless, those alternative tokens that aren’t lexical keywords are colloquially known as "digraphs".
Removal of trigraphs
Trigraphs were proposed for deprecation in C++0x, which was released as C++11. This was opposed by IBM, speaking on behalf of itself and other users of C++, and as a result trigraphs were retained in C++0x. Trigraphs were then proposed again for removal (not only deprecation) in C++17. This passed a committee vote, and trigraphs are expected to be removed from C++17 despite the opposition from IBM. Existing code that uses trigraphs can be supported by translating from the source files (parsing trigraphs) to the basic source character set that does not include trigraphs.
RPL
Hewlett-Packard calculators supporting the RPL language and input method provide support for a large number of trigraphs (also called TIO codes) to reliably transcribe non-seven-bit ASCII characters of the calculators' extended character set on foreign platforms, and to ease keyboard input without using the CHARS application.