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C + + : T h e C o m p l e t e R e f e r e n c e
You can apply the modifiers signed, short, long, and unsigned to integer base types.
You can apply unsigned and signed to characters. You may also apply long to double.
Table 2-1 shows all valid data type combinations, along with their minimal ranges and approximate bit widths. (These values also apply to a typical C++ implementation.)
Remember, the table shows the minimum range that these types will have as specified by Standard C/C++, not their typical range. For example, on computers that use two's complement arithmetic (which is nearly all), an integer will have a range of at least
32,767 to –32,768.
The use of signed on integers is allowed, but redundant because the default integer declaration assumes a signed number. The most important use of signed is to modify
char
in implementations in which char is unsigned by default.
The difference between signed and unsigned integers is in the way that the highorder bit of the integer is interpreted. If you specify a signed integer, the compiler generates code that assumes that the high-order bit of an integer is to be used as a
sign flag. If the sign flag is 0, the number is positive; if it is 1, the number is negative.
In general, negative numbers are represented using the two's complement approach, which reverses all bits in the number (except the sign flag), adds 1 to this number, and sets the sign flag to 1.
Signed integers are important for a great many algorithms, but they only have half the absolute magnitude of their unsigned relatives. For example, here is 32,767:
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
If the high-order bit were set to 1, the number would be interpreted as
−1. However, if you declare this to be an unsigned int, the number becomes 65,535 when the highorder bit is set to 1.
When a type modifier is used by itself (that is, when it does not precede a basic type), then int is assumed. Thus, the following sets of type specifiers are equivalent:
Specifier signed unsigned long short
Same As signed int unsigned int long int short int
Although the int is implied, many programmers specify the int anyway.
Identifier Names
In C/C++, the names of variables, functions, labels, and various other user-defined objects are called identifiers. These identifiers can vary from one to several characters.
