Your attention is invited to the fourth part of an e-book on undefined behavior. This is not a textbook, as it's intended for those who are already familiar with C++ programming. It's a kind of C++ programmer's guide to undefined behavior and to its most secret and exotic corners. The book was written by Dmitry Sviridkin and edited by Andrey Karpov.

C++ programmer's guide to undefined behavior: part 4 of 11

by Dmitry Sviridkin

From the article:

In the C++98, the committee made a terrible decision that seemed reasonable at the time. They created a specialization for std::vector<bool>. Normally, sizeof(bool) == sizeof(char), but one bit is enough for bool. However, 99.99% of all possible platforms can't address memory one bit at a time. Let's pack bits in vector<bool> and store CHAR_BIT (usually 8) boolean values in one byte (char) for more efficient memory utilization. As a result, one needs to work with std::vector<bool> in a very special way...