Backslashes denote two different things in C++, depending on the context.

As A Line Continuation

Outside of a quotes string (see below), a \ is used as a line continuation character. The newline that follows at the end of the line (not visible) is effectively ignored by the preprocessor and the following line is appended to the current line.

So:

s23_foo += \ 
s8_foo * s16_bar;

Is parsed as:

s23_foo += s8_foo * s16_bar;

Line continuations can be strung together. This:

s23_foo += \ 
s8_foo * \
s16_bar;

Becomes this:

s23_foo += s8_foo * s16_bar;

In C++ whitespace is irrelevant in most contexts, so in this particular example the line continuation is not needed. This should compile just fine:

s23_foo += 
s8_foo * s16_bar;

And in fact can be useful to help paginate the code when you have a long sequence of terms.

Since the preprocessor processed a #define until a newline is reached, line continuations are most useful in macro definitions. For example:

#define FOO() \
s23_foo += \ 
s8_foo * s16_bar; 

Without the line continuation character, FOO would be empty here.

As An Escape Sequence

Within a quotes string, a backslash is used as a delimiter to begin a 2-character escape sequence. For example:

"hello\n"

In this string literal, the \ begins an escape sequence, with the escape code being n. \n results in a newline character being embedded in the string. This of course means if you want a string to include the \ character, you have to escape that as well:

"hello\\there"

results in the string as viewed on the screen:

hello\there

The various escape sequences are documented here.

The backslash \ is a character, just like the letter A, the comma ,, and the number 4. In some programming languages, notably C and its descendants (and maybe ancestors), it is used inside a string or character literal to escape other characters. For instance, '\a' represents the bell character, and will produce a beep from the computer if you print it (printf("%c", '\a')).

As a C-language escape character, it is largely a human construct allowed by the compiler so humans can express, e.g., the bell character. The compiled code simply stores the character — a byte with the value 7. Just to be absolutely clear, it does not store a \ followed by an a.

Under other contexts, the backslash means something to the program at runtime. The most well-known instance of this is regular expression syntax, in which a backslash escape other characters in order to either give them special meaning or take away a special meaning they might have. For example, grep '\<foo\>' file.txt will locate lines with the word foo in file.txt. In this case the backslashes really are there at runtime, and are interpreted by the program as escapes for the < and > that follow them. In this case, \< and \> don't represent characters at all; they denote a zero-width match against the beginning and end of a word, respectively.

It's not an operator, really. It's just a line extension - it tells the preprocessor that the #define replacement text continues on the next line of the file.

Check out #3 at this link:

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.