411 REFERENCE · Wikibooks
C++ Language/Std/Stl/CollectionClasses/SequenceCollections/VectorOfBool
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A special specialization std::vector<bool> compresses each item to a single bit.
For this special specialization, the indexing operator doesn't return bool&.
Instead, it returns a reference to an auxillary class that defines both vecyVar[iIndex].flip() and a cast-operator for getting and setting its bool value.
Additional information about a vector of bool (includes interactive examples)