ISO/IEC JTC1 SC22 WG21
N4086
Richard Smith
2014-06-18

# Removing trigraphs??!

## Proposed wording

Change in 2.2 (lex.phases) paragraph 1 bullet 1:

> Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Trigraph sequences (2.4) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (2.3) is replaced by […]

Delete subclause 2.4 (lex.trigraph) "Trigraph sequences"

Change in 2.5 (lex.pptoken) paragraph 3 bullet 1:

> If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as `R"`, the next preprocessing token shall be a raw string literal. Between the initial and final double quote characters of the raw string, any transformations performed in phases 1 and 2 (trigraphs, universal-character-names, and line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified. […]

Change footnote 24 in 2.14.3 (lex.ccon) paragraph 3:

> Using an escape sequence for a question mark can avoid accidentally creating a trigraph is supported for compatibility with ISO C++14 and ISO C.

Add a subclause to Annex C:

> ### Clause 2: lexical conventions [diff.cpp14.lex]

> Change: Removal of trigraph support as a required feature.
> Rationale: Prevents accidental uses of trigraphs in non-raw string literals and comments.
> Effect on original feature: Valid C++2014 code that uses trigraphs may not be valid or may have different semantics. Implementations may choose to translate trigraphs as specified in C++2014 if they appear outside of a raw string literal, as part of the implementation-defined mapping from physical source file characters to the basic source character set.

## Feature test macro

No feature test macro is provided for this feature. Code that wishes to be portable between implementations that provide trigraphs and those that do not should avoid using basic source character sequences containing trigraphs.