C++ Types 3

Integer Types

Unsigned integer types

These types only vary in the amount of bits they use to represent numbers. The numbers are encoded in the bits as binary. C++03 mandates a set of unsigned integer types and a minimum number of bits and range for each type. The actual number of bits is implementation specific as long as it follows the following rules

1. (for the number of bits of each) unsigned char <= unsigned short int <= unsigned int <= unsigned long int.
2. char (and thus unsigned char) has enough bits to store the basic character set.
3. int has the “natural” bit size of the architecture (Usually 16, 32 or 64 bits)

It is interesting to note that the following condition is legal in a C++ implementation:

• The number of bits in the following types can all be equal: unsigned char = unsigned short int = unsigned int = unsigned long int (It would have to be at least 32 bits to satisfy the standard)
• For example the following is legal:

  Type	Bits	Range
  unsigned char	60	0-1152921504606846976
  unsigned short int	60	0-1152921504606846976
  unsigned int	60	0-1152921504606846976
  unsigned long int	60	0-1152921504606846976

Signed integer types

Signed integer types can be encoded in three different ways, sign magnitude, 1’s complement and 2’s complement. The C++03 standard allows an implementation to use any of these representations. However most implementations use 2’s complement. 2’s complement is really easy to understand, it’s just like the normal unsigned binary representation, except the highest digit represents the negation of the power of 2 of it’s position. For example 1011 means 1*(-8) + 0*4 + 1*2 + 1 = -8 + 2 + 1 = -5.

The signed types follow the same rules as the unsigned types (see above) and the ranges are as follows:

chars

There are some special rules for chars. In many ways char is the “base” type of a C++ system. It’s the smallest addressable set of bits when programming in C or C++. Other types’ sizes are always measured in terms of chars, sizeof(T) returns the number of chars needed to store T.

• char, signed char and unsigned char types must:
  • All have the same alignment requirements and have the same number of bits.
  • Large enough to represent the basic character set.
• char can either be signed or unsigned, it depends on the implementation.
  • VS2008 uses signed chars by default.