Integer literal

Allows values of integer type to be used in expressions directly.

Syntax

An integer literal has the form


decimal-literal integer-suffix (optional)	(1)

octal-literal integer-suffix (optional)	(2)

hex-literal integer-suffix (optional)	(3)

binary-literal integer-suffix (optional)	(4)	(since C++14)

where

decimal-literal is a non-zero decimal digit (1, 2, 3, 4, 5, 6, 7, 8, 9), followed by zero or more decimal digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
octal-literal is the digit zero (0) followed by zero or more octal digits (0, 1, 2, 3, 4, 5, 6, 7)
hex-literal is the character sequence 0x or the character sequence 0X followed by one or more hexadecimal digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, A, b, B, c, C, d, D, e, E, f, F)
binary-literal is the character sequence 0b or the character sequence 0B followed by one or more binary digits (0, 1)
integer-suffix, if provided, may contain one or both of the following (if both are provided, they may appear in any order:

unsigned-suffix (the character u or the character U)
one of

long-suffix (the character l or the character L)

long-long-suffix (the character sequence ll or the character sequence LL)

(since C++11)

size-suffix (the character z or the character Z)

(since C++23)

Optional single quotes (') may be inserted between the digits as a separator; they are ignored when determining the value of the literal.

(since C++14)

An integer literal (as any literal) is a primary expression.

Explanation

1) Decimal integer literal (base 10).

2) Octal integer literal (base 8).

3) Hexadecimal integer literal (base 16, the letters 'a' through 'f' represent values (decimal) 10 through 15).

4) Binary integer literal (base 2).

The first digit of an integer literal is the most significant.

Example. The following variables are initialized to the same value:

int d = 42;
int o = 052;
int x = 0x2a;
int X = 0X2A;
int b = 0b101010; // C++14

Example. The following variables are also initialized to the same value:

unsigned long long l1 = 18446744073709550592ull;       // C++11
unsigned long long l2 = 18'446'744'073'709'550'592llu; // C++14
unsigned long long l3 = 1844'6744'0737'0955'0592uLL;   // C++14
unsigned long long l4 = 184467'440737'0'95505'92LLU;   // C++14

The type of the literal

The type of the integer literal is the first type in which the value can fit, from the list of types which depends on which numeric base and which integer-suffix was used:

Suffix	Decimal bases	Binary, octal, or hexadecimal bases
(no suffix)	int long int long long int (since C++11)	int unsigned int long int unsigned long int long long int (since C++11) unsigned long long int (since C++11)
`u` or `U`	unsigned int unsigned long int unsigned long long int (since C++11)	unsigned int unsigned long int unsigned long long int (since C++11)
`l` or `L`	long int unsigned long int (until C++11) long long int (since C++11)	long int unsigned long int long long int (since C++11) unsigned long long int (since C++11)
both `l`/`L` and `u`/`U`	unsigned long int unsigned long long int (since C++11)	unsigned long int unsigned long long int (since C++11)
`ll` or `LL`	long long int (since C++11)	long long int (since C++11) unsigned long long int (since C++11)
both `ll`/`LL` and `u`/`U`	unsigned long long int (since C++11)	unsigned long long int (since C++11)
`z` or `Z`	the signed version of std::size_t (since C++23)	the signed version of std::size_t (since C++23) std::size_t (since C++23)
both `z`/`Z` and `u`/`U`	std::size_t (since C++23)	std::size_t (since C++23)

If the value of the integer literal that does not have size-suffix(since C++23) is too big to fit in any of the types allowed by suffix/base combination and the compiler supports an extended integer type (such as __int128) which can represent the value of the literal, the literal may be given that extended integer type — otherwise the program is ill-formed.

Notes

Letters in the integer literals are case-insensitive: 0xDeAdBeEfU and 0XdeadBEEFu represent the same number (one exception is the long-long-suffix, which is either ll or LL, never lL or Ll)(since C++11).

There are no negative integer literals. Expressions such as -1 apply the unary minus operator to the value represented by the literal, which may involve implicit type conversions.

In C prior to C99 (but not in C++), unsuffixed decimal values that do not fit in long int are allowed to have the type unsigned long int.

When used in a controlling expression of #if or #elif, all signed integer constants act as if they have type std::intmax_t and all unsigned integer constants act as if they have type std::uintmax_t.

(since C++11)

Due to maximal munch, hexadecimal integer literals ending in e and E, when followed by the operators + or -, must be separated from the operator with whitespace or parentheses in the source:

auto x = 0xE+2.0;   // error
auto y = 0xa+2.0;   // OK
auto z = 0xE +2.0;  // OK
auto q = (0xE)+2.0; // OK

Otherwise, a single invalid preprocessing number token is formed, which causes further analysis to fail.

Feature-test macro	Value	Std	Feature
`__cpp_binary_literals`	201304L	(C++14)	Binary literals
`__cpp_size_t_suffix`	202011L	(C++23)	Literal suffixes for std::size_t and its signed version

Example

Run this code

#include <cstddef>
#include <iostream>
#include <type_traits>
 
int main()
{
    std::cout << 123 << '\n'
              << 0123 << '\n'
              << 0x123 << '\n'
              << 0b10 << '\n'
              << 12345678901234567890ull << '\n'
              << 12345678901234567890u << '\n'; // the type is unsigned long long
                                                // even without a long long suffix
 
//  std::cout << -9223372036854775808 << '\n'; // error: the value
               // 9223372036854775808 cannot fit in signed long long, which is the
               // biggest type allowed for unsuffixed decimal integer literal
    std::cout << -9223372036854775808u << '\n'; // unary minus applied to unsigned
               // value subtracts it from 2^64, this gives 9223372036854775808
    std::cout << -9223372036854775807 - 1 << '\n'; // correct way to calculate
                                                   // the value -9223372036854775808
 
#if __cpp_size_t_suffix >= 202011L // C++23
    static_assert(std::is_same_v<decltype(0UZ), std::size_t>);
    static_assert(std::is_same_v<decltype(0Z), std::make_signed_t<std::size_t>>);
#endif
}

Output:

123
83
291
2
12345678901234567890
12345678901234567890
9223372036854775808
-9223372036854775808

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 2698	C++23	an integer literal with size-suffix could have an extended integer type	ill-formed if too large

References

C++23 standard (ISO/IEC 14882:2024):

5.13.2 Integer literals [lex.icon]

C++20 standard (ISO/IEC 14882:2020):

5.13.2 Integer literals [lex.icon]

C++17 standard (ISO/IEC 14882:2017):

5.13.2 Integer literals [lex.icon]

C++14 standard (ISO/IEC 14882:2014):

2.14.2 Integer literals [lex.icon]

C++11 standard (ISO/IEC 14882:2011):

2.14.2 Integer literals [lex.icon]

C++98 standard (ISO/IEC 14882:1998):

2.13.1 Integer literals [lex.icon]

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