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Numeric Literals

September 01, 2000

Dan_Saks-September 01, 2000





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Numeric Literals

Dan Saks

Every literal has a type.It may not be obvious, and it may vary across platforms,but the standard does specify it precisely.

The C language provides several different kinds of constants: integer constants such as 10 and 0x1C, floating constants such as 1.0 and 6.022e+23, and character constants such as 'a' and '\x10'. C also provides string literals such as "ouch!" and "\n". C++ provides the same kinds of constants and literals, except the C++ standard just calls all of them literals instead. I do, too.

Every literal in C and C++ has a type. For example, 10 is obviously an integer. But is it an int (which is signed by default) an unsigned int, or a short or long variation thereof? Do you even care? If you're a C programmer, you might not care. Part of me hopes you'd care, but I'm not absolutely sure you need to. Many C programmers apparently get by quite well without knowing the exact types of constants.

If you're a C++ programmer, you really should care. C++ supports function name overloading, and the exact type of a literal just might determine which function is the best match for a particular call. For example, given:

int f(int);
unsigned int f(unsigned int);
long int f(long int);

which of these functions does f(10) call? The answer is:

int f(int);

because the precise type of 10 is (signed)int.

Forms of integer literals
An integer literal takes different forms:

  • A decimal integer literal (base 10) is a non-zero digit followed by zero or more decimal digits
  • A hexadecimal integer literal (base sixteen) is 0x or 0X followed by a sequence of one or more hexadecimal digits
  • An octal integer literal (base eight) is the digit 0 followed by a sequence of zero or more octal digits

For example, the decimal integer literal 63 can also be expressed as the hexadecimal integer literal 0x3F or as the octal integer literal 077.

Any integer literal may have a suffix that influences its type:

U or u specifies that the literal has an unsigned type L or l specifies that the literal has a long type LL or ll specifies that the literal has a long long type. (This is available in the most recent edition of Standard C, C99, but not in earlier versions of C or in C++)

For example, 63u has type unsigned int. 0x3FL has type (signed)long int. An integer literal suffix may combine U with either L or LL (in either upper or lower case and in either order), so that both 63ul and 0x3FLU have type unsigned long int.

An integer literal's suffix influences the literal's type, but does not determine it. The type also depends on the literal's value.

Types of integer literals
In general, an integer literal's type is the smallest integer type no smaller than int that can hold the literal's value and still satisfy the constraints imposed by the form and suffix. For example, the type of a decimal integer literal with no suffix is the smallest (the first) of the following types that can represent its value: int, long int, and in C99, long long int. Because the maximum values of the types vary across platforms, the exact types for many integer literals also vary across platforms.

Both the C and C++ standards allow considerable slack in the range of values for integer types. The maximum value for an int must be at least 215-1, and the maximum value for a long int must be at least 231-1. In practice, this means that an int must occupy at least 16 bits and a long int must occupy at least 32 bits.

On any platform, int and long int may occupy more than their minimum storage requirement, as long as a long int occupies at least as much storage as an int. An unsigned integer type must occupy the same amount of storage as its corresponding signed integer type. For instance, a 16-bit platform might use 16 bits for int and unsigned int and 32 bits for long int and unsigned long int. A 32-bit platform might use 32 bits for all four types, int, unsigned int, long int, and unsigned long int.

Table 1 describes the rules for determining the type of an integer literal in C99 and C++. A literal whose value can't be represented in any of the listed types produces a compile error.



Click on image to enlarge.

The rules in Table 1 lead to some potential portability problems. A decimal integer literal whose value is not greater than 32,767 (= 215-1) has type int on every standard C and C++ environment. No surprise there. However, any decimal integer literal whose value is greater than 215-1 but not greater than 231-1 has type long int on a 16-bit platform, but just plain int on a 32-bit platform.

For example, given the overloaded functions:

int f(int);
unsigned int f(unsigned int);
long int f(long int);
unsigned long int
	f(unsigned long int);

calling f(32768) calls f(long int) when compiled using C++ for a 16-bit platform, but it calls f(int) when compiled for a 32-bit platform. On the other hand, if you write the call as f(0x8000) (0x8000 is 32,768 as a hexadecimal literal), it calls f(unsigned int) on every platform.

Floating literals
Floating literals offer a few little surprises, too. The type of a floating literal such as 1.0 or 6.022e+23 is double, not float. Appending the letter F or f to a floating literal makes its type float. Appending the letter L or l to a floating literal makes its type long double . For example, 1.0F has type float , and 6.022e+23L has type long double . Combining F and L (in either case and in either order) in a floating literal is a syntax error.

In C++, the only form of a floating literal is a decimal floating literal, in which all of the digits (in both the significant part and the exponent part) are decimal digits. C99 also provides hexadecimal floating literals, in which the digits of the significant part are hexadecimal digits.

In a decimal floating literal, the exponent part is optional. If present, it begins with E or e followed by decimal digits representing a power of 10. In a hexadecimal floating literal, the exponent part is mandatory. Since E and e are valid hexadecimal digits, a hexadecimal floating literal uses P or p to mark the beginning of the exponent part, following by decimal digits representing a power of two. For example, 0x1.FFFFFEp127f is a floating constant whose value is 0x1.FFFFFE multiplied by 2127 (decimal). The f at the end of the literal specifies that the literal's type is float.

Neither the form nor value of a floating literal affects its type. The suffix letter (or absence thereof) completely determines the type.

Character and string literals
Integer and floating literals are essentially the same in both C and C++. The differences lie in C's added support for long long integer types and hexadecimal floating literals. In contrast, character and string literals behave slightly differently between C and C++. Those differences are my topic for next time.

Dan Saks is the president of Saks & Associates, a C/C++ training and consulting company. He is also a consulting editor for the C/C++ Users Journal. He served for many years as secretary of the C++ standards committee. With Thomas Plum, he wrote C++ Programming Guidelines (Plum Hall).

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