Literals for numbers, characters and strings
Remarks#
The term literal is commonly used to describe a sequence of characters in a C code that designates a constant value such as a number (e.g. 0) or a string (e.g. "C"). Strictly speaking, the standard uses the term constant for integer constants, floating constants, enumeration constants and character constants, reserving the term ‘literal’ for string literals, but this is not common usage.
Literals can have prefixes or suffixes (but not both) which are extra characters that can start or end a literal to change its default type or its representation.
Integer literals
Integer literals are used to provide integral values. Three numerical bases are supported, indicated by prefixes:
| Base | Prefix | Example |
|---|---|---|
| Decimal | None | 5 |
| Octal | 0 |
0345 |
| Hexadecimal | 0x or 0X |
0x12AB, 0X12AB, 0x12ab, 0x12Ab |
Note that this writing doesn’t include any sign, so integer literals are always positive. Something like -1 is treated as an expression that has one integer literal (1) that is negated with a -
The type of a decimal integer literal is the first data type that can fit the value from int and long. Since C99, long long is also supported for very large literals.
The type of an octal or hexadecimal integer literal is the first data type that can fit the value from int, unsigned, long, and unsigned long. Since C99, long long and unsigned long long are also supported for very large literals.
Using various suffixes, the default type of a literal can be changed.
| Suffix | Explanation |
|---|---|
L, l |
long int |
LL, ll (since C99) |
long long int |
U, u |
unsigned |
The U and L/LL suffixes can be combined in any order and case. It is an error to duplicate suffixes (e.g. provide two U suffixes) even if they have different cases.
String literals
String literals are used to specify arrays of characters. They are sequences of characters enclosed within double quotes (e.g. "abcd" and have the type char*).
The L prefix makes the literal a wide character array, of type wchar_t*. For example, L"abcd".
Since C11, there are other encoding prefixes, similar to L:
| prefix | base type | encoding |
|---|---|---|
| none | char |
platform dependent |
L |
wchar_t |
platform dependent |
u8 |
char |
UTF-8 |
u |
char16_t |
usually UTF-16 |
U |
char32_t |
usually UTF-32 |
For the latter two, it can be queried with feature test macros if the encoding is effectively the corresponding UTF encoding.
Floating point literals
Floating point literals are used to represent signed real numbers. The following suffixes can be used to specify type of a literal:
| Suffix | Type | Examples |
|---|---|---|
| none | double |
3.1415926 -3E6 |
f, F |
float |
3.1415926f 2.1E-6F |
l, L |
long double |
3.1415926L 1E126L |
In order to use these suffixes, the literal must be a floating point literal. For example, 3f is an error, since 3 is an integer literal, while 3.f or 3.0f are correct. For long double, the recommendation is to always use capital L for the sake of readability.
Character literals
Character literals are a special type of integer literals that are used to represent one character. They are enclosed in single quotes, e.g. 'a' and have the type int. The value of the literal is an integer value according to the machine’s character set. They do not allow suffixes.
The L prefix before a character literal makes it a wide character of type wchar_t. Likewise since C11 u and U prefixes make it wide characters of type char16_t and char32_t, respectively.
When intending to represent certain special characters, such as a character that is non-printing, escape sequences are used. Escape sequences use a sequence of characters that are translated into another character. All escape sequences consist of two or more characters, the first of which is a backslash \. The characters immediately following the backslash determine what character literal the sequence is interpreted as.
| Escape Sequence | Represented Character |
|---|---|
\b |
Backspace |
\f |
Form feed |
\n |
Line feed (new line) |
\r |
Carriage return |
\t |
Horizontal tab |
\v |
Vertical tab |
\\ |
Backslash |
\' |
Single quotation mark |
\" |
Double quotation mark |
\? |
Question mark |
\nnn |
Octal value |
\xnn… |
Hexadecimal value |
| Escape Sequence | Represented Character |
|---|---|
\a |
Alert (beep, bell) |
| Escape Sequence | Represented Character |
|---|---|
\unnnn |
Universal character name |
\Unnnnnnnn |
Universal character name |
A universal character name is a Unicode code point. A universal character name may map to more than one character. The digits n are interpreted as hexadecimal digits. Depending on the UTF encoding in use, a universal character name sequence may result in a code point that consists of multiple characters, instead of a single normal char character.
When using the line feed escape sequence in text mode I/O, it is converted to the OS-specific newline byte or byte sequence.
The question mark escape sequence is used to avoid trigraphs. For example, ??/ is compiled as the trigraph representing a backslash character '\', but using ?\?/ would result in the string "??/".
There may be one, two or three octal numerals n in the octal value escape sequence.