Why does this crash (compilable code included)?

[d_stranz]

but in initialization list, calling _bitmastFromInternal(mask) means calling copy constructor of QBitArray

Wrong. All you are doing is storing a reference to the QBitArray, because your member variable (_bitmaskFromInternal) is also a reference. If you want to make a copy, declare _bitmaskFromInternal as a QBitArray, not a QBitArray &.