Integer literals

Note: C++0x is a new version of the C++ programming language standard. This is a draft standard and has not been officially adopted in its entirety. The implementation of C++0x is based on IBM's interpretation of the draft C++0x standard and is subject to change at any time without notice. IBM makes no attempt to maintain compatibility with earlier releases and therefore the C++0x language extension should not be relied on as a stable programming interface.

Integer literals are numbers that do not have a decimal point or an exponential part. They can be represented as:

Decimal integer literals
Hexadecimal integer literals
Octal integer literals

An integer literal might have a prefix that specifies its base, or a suffix that specifies its type.


Integer literal syntax

>>-+-decimal_constant-----+--+---------------+-----------------><
   +-octal_constant-------+  +-+-l--+--+---+-+   
   '-hexadecimal_constant-'  | +-L--+  +-u-+ |   
                             | +-ll-+  '-U-' |   
                             | '-LL-'        |   
                             '-+-u-+--+----+-'   
                               '-U-'  +-l--+     
                                      +-L--+     
                                      +-ll-+     
                                      '-LL-'

The `long long` features

There are two long long features:

the C99 long long feature
the non-C99 long long feature

Both of the two features have the corresponding extension parts:

the C99 long long feature with the associated IBM extensions
the non-C99 IBM long long extension

Types of integer literals outside of C99 and C++0x

C only In the non-C99 modes, you can enable the non-C99 IBM long long extension.

C++ only When the C99 long long feature is not in effect, you can enable the non-C99 IBM long long extension.

The following table lists the integer literals and shows the possible data types when the C99 long long feature is not enabled.

Table 1. Types of integer literals outside of C99 and C++0x
Representation	Suffix	Promotion order
		int	unsigned int	long int	unsigned long int	long long int	unsigned long long int
decimal	None	+		+	+¹
octal, hex	None	+	+	+	+
all	`u` or `U`		+		+
decimal	`l` or `L`			+	+¹
octal, hex	`l` or `L`			+	+
all	Both `u` or `U` and `l` or `L`				+
decimal	`ll` or `LL`					+	+
octal, hex	`ll` or `LL`					+	+
all	Both `u` or `U` and `ll` or `LL`						+
Notes: The `unsigned long int` type is not included here in the C++98 and C++03 standards. The C++ compiler includes the type in the implementation for compatibility purposes only. When none of the `long long` features are enabled, types of integer literals include all the types in this table except the last two columns.

Types of integer literals in C99 and C++0x

C only In the C99 modes, the C99 long long feature is enabled automatically.

C++ only When the non-C99 IBM long long extension is not in effect, you can enable the C99 long long feature.

After you enable the C99 long long feature, the compiler has all the functionality of the non-C99 IBM long long extension. Aside from literals that are out of range, the only difference is the specific typing rules for decimal integer literals that do not have a suffix containing u or U. Literals that are out of range under the non-C99 IBM long long extension might have implied type long long int or unsigned long long int under the C99 long long feature with the associated IBM extensions.

The following example demonstrates the different behaviors of the compiler when you use these two long long modes:

#include <stdio.h>

int main(){
    if(0>3999999999-4000000000){
        printf("C99 long long");
    }
    else{
        printf("non-C99 IBM long long extension");
    }
}

In this example, the values 3999999999 and 4000000000 are too large to fit into the a 32-bit long int type, but they can fit into either the unsigned long or the long long int type. If you enable the C99 long long feature, the two values have the long long int type, so the difference of 3999999999 and 4000000000 is negative. Otherwise, if you enable the non-C99 IBM long long extension, the two values have the unsigned long type, so the difference is positive.

When both the C99 and non-C99 long long features are disabled, integer literals that have one of the following suffixes cause a severe compiler error:

ll or LL
Both u or U and ll or LL

IBM extension If a value cannot fit into the long long int type, the compiler might use the unsigned long long int type for the literal. In this case, the compiler generates a message to indicate that the value is too large.

C++0x IBM extension To strictly conform to the C++0x standard, the compiler introduces the extended integer safe behavior to ensure that a signed value never becomes an unsigned value after a promotion. After you enable this behavior, if a decimal integer literal that does not have a suffix containing u or U cannot be represented by the long long int type, the compiler issues an error message to indicate that the value of the literal is out of range. The extended integer safe behavior is the only difference between the C99 long long feature with the associated IBM extensions and the C99 long long feature.

The following table lists the integer literals and shows the possible data types when the C99 long long feature is enabled.

Table 2. Types of integer literals in C99 and C++0x
Representation	Suffix	Promotion order
		int	unsigned int	long int	unsigned long int	long long int	unsigned long long int
decimal	None	+		+		+	+¹
octal, hex	None	+	+	+	+	+	+
all	`u` or `U`		+		+		+
decimal	`l` or `L`			+		+	+¹
octal, hex	`l` or `L`			+	+	+	+
all	Both `u` or `U` and `l` or `L`				+		+
decimal	`ll` or `LL`					+	+¹
octal, hex	`ll` or `LL`					+	+
all	Both `u` or `U` and `ll` or `LL`						+
Note: The compiler does not support this type if the extended integer safe behavior is enabled.

Decimal integer literals

A decimal integer literal contains any of the digits 0 through 9. The first digit cannot be 0. Integer literals beginning with the digit 0 are interpreted as an octal integer literal rather than as a decimal integer literal.


Decimal integer literal syntax

                 .--------------.   
                 V              |   
>>-digit_1_to_9----digit_0_to_9-+------------------------------><

The following are examples of decimal literals:

485976
5

A plus (+) or minus (-) symbol can precede a decimal integer literal. The operator is treated as a unary operator rather than as part of the literal. Consider the following example:

-433132211
+20

Hexadecimal integer literals

A hexadecimal integer literal begins with the 0 digit followed by either an x or X, followed by any combination of the digits 0 through 9 and the letters a through f or A through F. The letters A (or a) through F (or f) represent the values 10 through 15, respectively.


Hexadecimal integer literal syntax

           .------------------.   
           V                  |   
>>-+-0x-+----+-digit_0_to_f-+-+--------------------------------><
   '-0X-'    '-digit_0_to_F-'

The following are examples of hexadecimal integer literals:

0x3b24 
0XF96 
0x21 
0x3AA 
0X29b 
0X4bD

Octal integer literals

An octal integer literal begins with the digit 0 and contains any of the digits 0 through 7.


Octal integer literal syntax

      .--------------.   
      V              |   
>>-0----digit_0_to_7-+-----------------------------------------><

The following are examples of octal integer literals: