An escape sequence is a sequence of characters that are to be replaced by another character sequence. This is necessary because some characters have special meaning to the language compiler and therefore cannot be used as character literals. For example, the double quotation character (") is used to delimit literal strings, but if we want to include a double quotation mark in our string then we must use an escape sequence to tell the compiler that it is not to be treated as a delimiter. The escape sequence for a double-quote is ", and can be used as follows:

char str[] = ''The man said, "Hello world!"";

All escape sequences are prefixed with a backslash character which means the backslash itself has special meaning as character literal. So if we want to include a backslash in our string then we must use the double backslash escape sequence, \\, like so:

char str[] = ''c:\\temp\\file.tmp";

Other escape sequences are mostly used to represent special characters that have no simple representation on the keyboard, such as backspace and newline. The C++ standard defines the following escape sequences:

\' - single quote (ASCII character code 0x27)

" - double quote (ASCII character code 0x22)

\? - question mark (ASCII character code 0x3f)

\\ - backslash (ASCII character code 0x5c)

\a - audible bell (a system beep) (ASCII character code 0x07)

\b - backspace (ASCII character code 0x08)

\f - form feed (ASCII character code 0x0c)

\n - newline (ASCII character code 0x0a)

\r - carriage return (ASCII character code 0x0d)

\t - horizontal tab (ASCII character code 0x09)

\v - vertical tab (ASCII character code 0x0b)

\nnn - arbitrary octal value (byte nnn)

\Xnn - arbitrary hexadecimal value (byte nn)

\Unnnn - universal character code (code point U+nnnn)

\Unnnnnnnn - universal character code (code point U+nnnnnnnn)

Of the octal escape sequences, \0 is the most useful as this represents the null character (ASCII character code 0x00). Note that octal escape sequences have a limit if 3 digits, but will terminate at the first non-octal digit if encountered sooner.

Hexadecimal escape sequences have no length limit, but will terminate at the first non-hexadecimal digit. However, if a hexadecimal value exceeds the limit of the character width (such as char or wchar_t) the result is unspecified.

The single quote escape sequence is not usually required in strings, but it is required when used as a character literal because a single quote delimits all character literals:

char quot = '\'';

The question mark is seldom required unless a string contains a trigraph such as "??/". This trigraph is interpreted as meaning "" (the same as the "\" escape sequence) whereas "?\?/" would be interpreted as being the trigraph itself "??/".

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