The Unicode Standard is the specification of an encoding scheme for written characters and text. It is a universal standard that enables consistent encoding of multilingual text and allows text data to be interchanged internationally without conflict. The ISO standards for C and C++ refer to Information technology - Programming Languages - Universal Multiple-Octet Coded Character Set (UCS), ISO/IEC 10646:2003. (The term octet is used by ISO to refer to a byte.) The ISO/IEC 10646 standard is more restrictive than the Unicode Standard in the number of encoding forms: a character set that conforms to ISO/IEC 10646 is also conformant to the Unicode Standard.
The Unicode Standard specifies a unique numeric value and name for each character and defines three encoding forms for the bit representation of the numeric value. The name/value pair creates an identity for a character. The hexadecimal value representing a character is called a code point. The specification also describes overall character properties, such as case, directionality, alphabetic properties, and other semantic information for each character. Modeled on ASCII, the Unicode Standard treats alphabetic characters, ideographic characters, and symbols, and allows implementation-defined character codes in reserved code point ranges. According to the Unicode Standard, the encoding scheme of the standard is therefore sufficiently flexible to handle all known character encoding requirements, including coverage of all the world's historical scripts.
C99 and C++ allow the universal character name construct defined in ISO/IEC 10646 to represent characters outside the basic source character set. Both languages permit universal character names in identifiers, character constants, and string literals.
The following table shows the generic universal character name construct and how it corresponds to the ISO/IEC 10646 short name.
| Universal character name | ISO/IEC 10646 short name |
|---|---|
| where N is a hexadecimal digit | |
| \UNNNNNNNN | NNNNNNNN |
| \uNNNN | 0000NNNN |
C99 and C++ disallow the hexadecimal values representing characters in the basic character set (base source code set) and the code points reserved by ISO/IEC 10646 for control characters.
The following characters are also disallowed: