gcc multi line comment warning

Question

I'm getting a warning

warning: multi-line comment [-Wcomment]

due to a comment that I have that looks something like

// strings can start with a \ and also end with a \ 

I understand the error and have seen other SO messages on the subject.

I can easily fix the warning (by double quoting the \\s).

What I'm curious about is that I took care to make sure that the line does not end with a \\. The line ends with \\ and then a space. Is this the preprocessor stripping my trailing space and thereby introducing the warning?

Answer 1

During the initial processing preprocessor performs a series of textual transformations on its input.

Here's the quote from the docs (relevant piece is in bold):

Continued lines are merged into one long line.

A continued line is a line which ends with a backslash, . The backslash is removed and the following line is joined with the current one.

...

The trailing backslash on a continued line is commonly referred to as a backslash-newline.

If there is white space between a backslash and the end of a line, that is still a continued line. However, as this is usually the result of an editing mistake, and many compilers will not accept it as a continued line, GCC will warn you about it.

In this case it is best to use '\\' instead of \\ as backslash is used as a symbol and not as a continued-line indicator. Another (subjectively inferior) option is to put an ending non-whitespace character after \\ (for example a dot).