Digraphs
Not some punctuation marks are not available on all keyboards. In this case we can use the following Digraphs in our source files.
<: is translated into [
:> is translated into ]
<% is translated into \{
%> is translated into \}
%: is translated into \#
%:%: is translated into \##
The plain idea
Digraphs, like trigraphs, are alternate spellings for punctuation that some keyboards lacked, this time two characters instead of three. <: means [, %> means }, and so on. The crucial difference from trigraphs is that digraphs are real tokens: the compiler treats <: as the [ token, but it never reaches inside a string literal to change your text.
Why it matters
They are safer than trigraphs for exactly that reason, and unlike trigraphs they are still part of C and C++. You will rarely write them, but you may meet them in old code, and knowing they are harmless, equivalent spellings saves confusion.
An everyday analogy: spelling “and” as “&”
Writing “fish & chips” instead of “fish and chips” is a shorthand everyone understands, and it never accidentally changes a word inside a sentence. Digraphs are that kind of tidy substitution: an alternate spelling at the level of whole symbols, not stray letters.
A worked example
These two lines compile to exactly the same program:
int a<:3:> = <%1, 2, 3%>; // with digraphs
int a[3] = { 1, 2, 3 }; // the normal way
Because <: is a token, this works cleanly, whereas the trigraph version would risk mangling anything that happened to contain the sequence.
Common misconceptions
- “Digraphs and trigraphs are interchangeable.” They solve the same problem but differ in a key way: digraphs are tokens (string-safe, still in the language); trigraphs were raw character substitutions (string-unsafe, removed in C23).
- “They slow compilation or change behaviour.” No; they are purely alternate spellings with identical meaning.
How it connects
- Trigraphs: the three-character, now-removed cousin.
- Tokens and The Compiler: digraphs are recognised during tokenizing.
The truth revealed: digraphs are the civilised version of the keyboard-shortage fix, alternate spellings that mean exactly what they look like and never surprise you.