vector::operator[] misbehavior? [duplicate]

Answer 1

vector<bool>::operator[] neither yields a bool nor a reference to a bool. It just returns a little proxy object that acts like a reference. This is because there are no references to single bits and vector<bool> actually stores the bools in a compressed way. So by using auto you just created a copy of that reference-like object. The problem is that C++ does not know that this object acts as a reference. You have to force the "decay to a value" here by replacing auto with T.

Answer 2

operator[] returns a T& for every value of T except for bool, where it gives a reference proxy. See this old column by Herb Sutter on why using vector<bool> in generic code is bad idea (and why it is not even a container). There is also a special Item about it in Effective STL by Scott Meyers, and tons of questions on it here at SO.

Answer 3

vector<bool> is a hideous abomination and special. The Committee specialized it to pack bits, therefore it does not support proper reference semantics, as you cannot refer to a bit, this means that it has a non-conforming interface and does not actually qualify as a Standard Container. The solution that most people use is simply to never, ever, use vector<bool>.