libstdc++
std::tr2::dynamic_bitset< _WordT, _Alloc > Class Template Reference
Inheritance diagram for std::tr2::dynamic_bitset< _WordT, _Alloc >:
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Detailed Description

template<typename _WordT = unsigned long long, typename _Alloc = std::allocator<_WordT>>
class std::tr2::dynamic_bitset< _WordT, _Alloc >

The dynamic_bitset class represents a sequence of bits.

See N2050, Proposal to Add a Dynamically Sizeable Bitset to the Standard Library.

In the general unoptimized case, storage is allocated in word-sized blocks. Let B be the number of bits in a word, then (Nb+(B-1))/B words will be used for storage. B - NbB bits are unused. (They are the high-order bits in the highest word.) It is a class invariant that those unused bits are always zero.

If you think of dynamic_bitset as "a simple array of bits," be aware that your mental picture is reversed: a dynamic_bitset behaves the same way as bits in integers do, with the bit at index 0 in the "least significant / right-hand" position, and the bit at index Nb-1 in the "most significant / left-hand" position. Thus, unlike other containers, a dynamic_bitset's index "counts from right to left," to put it very loosely.

This behavior is preserved when translating to and from strings. For example, the first line of the following program probably prints "b('a') is 0001100001" on a modern ASCII system.

#include <dynamic_bitset>
#include <iostream>
#include <sstream>
using namespace std;
int main()
{
long a = 'a';
dynamic_bitset<> b(a);
cout << "b('a') is " << b << endl;
s << b;
string str = s.str();
cout << "index 3 in the string is " << str[3] << " but\n"
<< "index 3 in the bitset is " << b[3] << endl;
}

Most of the actual code isn't contained in dynamic_bitset<> itself, but in the base class __dynamic_bitset_base. The base class works with whole words, not with individual bits. This allows us to specialize __dynamic_bitset_base for the important special case where the dynamic_bitset is only a single word.

Extra confusion can result due to the fact that the storage for __dynamic_bitset_base is a vector, and is indexed as such. This is carefully encapsulated.

Definition at line 413 of file dynamic_bitset.


The documentation for this class was generated from the following file: