vector<bool> doesn’t behave like other STL containers. If you try to obtain a reference to a bool inside the container, you’ll end up with an rvalue. Probably not what you had in mind. Oddly, it IS standard compliant (more on this later). Therefore you can’t say it’s not “standard compliant” container. What it doesn’t follow is the optional sequence container operations format from table 101 in section [sequence.reqmts / 23.2.3]. This table states the format for operator [] to return a reference or a const_reference. Assuming in this case vector<T>, a reference would be a T& and a const_reference a const T&. In the case of vector<bool>, operator[] returns a “reference” which is a class (more on this later). So there you have it. vector<bool> is standards compliant, but doesn’t meet the container requirements.