rspec/rules/S797/cfamily/rule.adoc

== Why is this an issue?

Trigraphs are denoted by a sequence of 2 question marks followed by a specified third character (e.g. ??- represents a '~' (tilde) character and ??) represents a ']'). They can cause accidental confusion with other uses of two question marks.


=== Noncompliant code example

[source,cpp]
----
static const char str[] = "(Date should be in the form ??-??-??)"; // Noncompliant. Evaluates to "(Date should be in the form ~~]"
----


=== Compliant solution

[source,cpp]
----
static const char str[] = "(Date should be in the form ?" "?-?" "?-?" ?)";  // adjacent string literals concatenated at compile time
static const char str2[] = "(Date should be in the form ?-?-?)"; // problem avoided by eliminating 2nd '?' in each sequence
static const char str3[] = "(Date should be in the form ? ?-? ?-? ?)"; // problem avoided by spacing '?'s out
----


== Resources

=== Documentation

* {cpp} reference - https://en.cppreference.com/w/cpp/language/operator_alternative[Alternative operator representations: Trigraphs]

=== Standards

* CERT - https://wiki.sei.cmu.edu/confluence/x/uNUxBQ[PRE07-C. - Avoid using repeated question marks]

=== External coding guidelines

* MISRA C:2004, 4.2 - Trigraphs shall not be used
* MISRA {cpp}:2008, 2-3-1 - Trigraphs shall not be used
* MISRA C:2012, 4.2 - Trigraphs shall not be used


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Remove all trigraph sequences: ??=, ??/, ??', ??(, ??), ??!, ??<, ??>, ??-.


endif::env-github,rspecator-view[]