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Thinking in C++ Vol 2 - Practical Programming
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### vector<bool>

The vector<bool> container is a specialization of the vector template. A normal bool variable requires at least one byte, but since a bool only has two states, the ideal implementation of vector<bool> is such that each bool value only requires one bit. Since typical library implementations pack the bits into integral arrays, the iterator must be specially defined and cannot be a pointer to bool.

The bit-manipulation functions for vector<bool> are much more limited than those of bitset. The only member function that was added to those already in vector is flip( ), to invert all the bits. There is no set( ) or reset( ) as in bitset. When you use operator[ ], you get back an object of type vector<bool>::reference, which also has a flip( ) to invert that individual bit.

//: C07:VectorOfBool.cpp
// Demonstrate the vector<bool> specialization.
#include <bitset>
#include <cstddef>
#include <iostream>
#include <iterator>
#include <sstream>
#include <vector>
using namespace std;

int main() {
vector<bool> vb(10, true);
vector<bool>::iterator it;
for(it = vb.begin(); it != vb.end(); it++)
cout << *it;
cout << endl;
vb.push_back(false);
ostream_iterator<bool> out(cout, "");
copy(vb.begin(), vb.end(), out);
cout << endl;
bool ab[] = { true, false, false, true, true,
true, true, false, false, true };
// There's a similar constructor:
vb.assign(ab, ab + sizeof(ab)/sizeof(bool));
copy(vb.begin(), vb.end(), out);
cout << endl;
vb.flip(); // Flip all bits
copy(vb.begin(), vb.end(), out);
cout << endl;
for(size_t i = 0; i < vb.size(); i++)
vb[i] = 0; // (Equivalent to "false")
vb[4] = true;
vb[5] = 1;
vb[7].flip(); // Invert one bit
copy(vb.begin(), vb.end(), out);
cout << endl;
// Convert to a bitset:
ostringstream os;
copy(vb.begin(), vb.end(),
ostream_iterator<bool>(os, ""));
bitset<10> bs(os.str());
cout << "Bitset: << endl << bs << endl;
} ///:~

The last part of this example takes a vector<bool> and converts it to a bitset by first turning it into a string of ones and zeros. Here, you must know the size of the bitset at compile time. You can see that this conversion is not the kind of operation you ll want to do on a regular basis.

The vector<bool> specialization is a crippled STL container in the sense that certain guarantees that other containers provide are missing. For example, with the other containers the following relationships hold:

// Let c be an STL container other than vector<bool>:
T& r = c.front();
T* p = &*c.begin();

For all other containers, the front( ) function yields an lvalue (something you can get a non-const reference to), and begin( ) must yield something you can dereference and then take the address of. Neither is possible because bits are not addressable. Both vector<bool> and bitset use a proxy class (the nested reference class, mentioned earlier) to read and set bits as necessary.

Thinking in C++ Vol 2 - Practical Programming
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 Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire