C++ single line comments followed by \ transforms in multiline comment

Question

Where is it documented in the C++ Standard the feature that if a line is commented using //some comment\ style (at the end of the line puts \) the comment is transformed to multiline?

Tested with g++ 4.8 and VS 2012

//some interesting stuff\
another interesting stuff\
etc

Answer 1

C++ standard, 2.2 - phases of translation. Phase 2 includes

Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines.

and Phase 3 includes

Each comment is replaced by one space character

So the backslash at the end of the line is recognised before comments.

Equivalent phases 2 and 3 for C can be found in C standard (5.1.1.2 Translation phases in my draft).

Answer 2

A \ followed by a new line is eliminated very early in the translation process, before the compiler starts looking for comments and the end of comments, see §2.2, Phases of translation.

Answer 3

Do you want to know for C or C++? (EDIT: In original question OP asked for C/C++)

For C following section from ISO/IEC 9899:TC2 Committee Draft — May 6, 2005 WG14/N1124 answers your question.

5.1.1.2 Translation phases

[2] Each instance of a backslash character () immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.

For C++, you can refer to Phase 2 at en.cppreference.com

1) Whenever backslash appears at the end of a line (immediately followed by the newline character), both backslash and newline are deleted, combining two physical source lines into one logical source line. This is a single-pass operation, a line ending in two backslashes followed by an empty line does not combine three lines into one). If a universal character name (\uXXX) is formed on this phase, the behavior is undefined.
2) If a non-empty source file does not end with a newline character after this step (whether it had no newline originally, or it ended with a backslash) the behavior is undefined (until C++11) a terminating newline character is added (since C++11)

If your current line is a single line comment, following line would be digested in continuation as a comment.

Answer 4

http://www.cplusplus.com/forum/general/33653/

You can add the "\" anywhere in the code and newline will be ignored.

As a better reference the 2.2 paragraph of the standard:

Each instance of a backslash character () immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a new-line character immediately preceded by a backslash character before any such splicing takes place, shall be processed as if an additional new-line character were appended to the file.

This is not clear regarding what happens if the last character in the file is a backslash. In such a case, presumably the result of adding the newline should not be a line splice but rather a backslash preprocessing-token (that will be diagnosed as an invalid token in phase 7), but that should be spelled out.