Bit Array - Language Support

Language Support

The C programming language's bitfields, pseudo-objects found in structs with size equal to some number of bits, are in fact small bit arrays; they are limited in that they cannot span words. Although they give a convenient syntax, the bits are still accessed using bitwise operators on most machines, and they can only be defined statically (like C's static arrays, their sizes are fixed at compile-time). It is also a common idiom for C programmers to use words as small bit arrays and access bits of them using bit operators. A widely available header file included in the X11 system, xtrapbits.h, is “a portable way for systems to define bit field manipulation of arrays of bits.” A more explanatory description of aforementioned approach can be found in the comp.lang.c faq.

In C++, although individual bools typically occupy the same space as a byte or an integer, the STL type vector is a partial template specialization in which bits are packed as a space efficiency optimization. Since bytes (and not bits) are the smallest addressable unit in C++, the operator does not return a reference to an element, but instead returns a proxy reference. This might seem a minor point, but it means that vector is not a standard STL container, which is why the use of vector is generally discouraged. Another unique STL class, bitset, creates a vector of bits fixed at a particular size at compile-time, and in its interface and syntax more resembles the idiomatic use of words as bit sets by C programmers. It also has some additional power, such as the ability to efficiently count the number of bits that are set. The Boost C++ Libraries provide a dynamic_bitset class whose size is specified at run-time.

The D programming language provides bit arrays in both of its competing standard libraries. In Phobos, they are provided in std.bitmanip, and in Tango, they are provided in tango.core.BitArray. As in C++, the operator does not return a reference, since individual bits are not directly addressable on most hardware, but instead returns a bool.

In Java, the class BitSet creates a bit array that is then manipulated with functions named after bitwise operators familiar to C programmers. Unlike the bitset in C++, the Java BitSet does not have a "size" state (it has an effectively infinite size, initialized with 0 bits); a bit can be set or tested at any index. In addition, there is a class EnumSet, which represents a Set of values of an enumerated type internally as a bit vector, as a safer alternative to bitfields.

The .NET Framework supplies a BitArray collection class. It stores boolean values, supports random access and bitwise operators, can be iterated over, and its Length property can be changed to grow or truncate it.

Although Standard ML has no support for bit arrays, Standard ML of New Jersey has an extension, the BitArray structure, in its SML/NJ Library. It is not fixed in size and supports set operations and bit operations, including, unusually, shift operations.

Haskell likewise currently lacks standard support for bitwise operations, but both GHC and Hugs provide a Data.Bits module with assorted bitwise functions and operators, including shift and rotate operations and an "unboxed" array over boolean values may be used to model a Bit array, although this lacks support from the former module.

In Perl, strings can be used as expandable bit arrays. They can be manipulated using the usual bitwise operators (~ | & ^), and individual bits can be tested and set using the vec function.

In Ruby, you can access (but not set) a bit of an integer (Fixnum or Bignum) using the bracket operator (), as if it were an array of bits.

Apple's Core Foundation library contains CFBitVector and CFMutableBitVector structures.

PL/I supports arrays of bit strings of arbitrary length, which may be either fixed-length or varying. The array elements may be aligned— each element begins on a byte or word boundary— or unaligned— elements immediately follow each other with no padding.

Hardware description languages such as VHDL, Verilog, and SystemVerilog natively support bit vectors as these are used to model storage elements like flip-flops, hardware busses and hardware signals in general. In hardware verification languages such as OpenVera, e and SystemVerilog, bit vectors are used to sample values from the hardware models, and to represent data that is transferred to hardware during simulations.