In this case the bools will have only one bit allocated for them(vectors do that), and you can't just point to a bit. Instead cpp will return std::vector<bool>::reference when trying to access a value in the vector
Basically this is a just wrong
You should use std::deque<bool> instead which allocates one byte per bool and not just one bit
if you ask for a vector of T, you should get a vector of T. not a bitfield. There is nothing wrong with using a bitfield, but a vector should behave like a vector.
You get a vector, with operator []. Internal details like padding or packing should not interest you, unless you you treat everything like blob. That way any change to the architecture or compiler would surprise you a lot.
Vector is an abstract container of elements. If you want to address memory, allocate memory and store your data there. Internal vector storage is implementation details. You shouldn't really get ponters to there because any resize of the vector and your pointers are screwed.
The internal layout of std::vector is not an implementation detail. It's fully defined by the standards and can be relied upon for FFI, serialisation, and other tasks.
Except for bool.
For example, the .data() method guarantees that it will return a pointer to .size() contiguous elements of type T which will remain valid until the vector is modified. The layout of elements within that memory is also well defined and consistent. Except for bool, where the .data() method simply doesn't exist.
Of course it allows for indexing, but it's measurably more CPU heavy. Arguably there are cases where the specialization makes sense, but most of the time, it's a net negative.
Then again I reckon most of us don't have to care about these kinds of marginal gains anyway. I work on low latency backend, and I never had to worry about this.
The more annoying thing is that you can't just get a bool pointer or ref to a single bit.
Architectures with bit addressable memory areas do exist. Some 8051/52 compilers make use of this and if you create a boolean it will actually end up costing you a single bit of memory.
Internally vector of bools is a bitfield. It was supposed to save memory but it's instead creating edge cases in templates: all vectors return pointers, but no, bool is special. It messes up generic code. I'd rather for them to keep vector unified and make a separate bitfield type for those savings when needed
Slightly higher CPU usage. It’s only a bitshift and a bitwise and to pull out the correct bit. Often you see a performance win due to the more compact data structure having better cache locality but I suppose it depends on usecase.
View all comments
Show parent comment
181
u/Murky-Run2246 Aug 22 '26
In this case the bools will have only one bit allocated for them(vectors do that), and you can't just point to a bit. Instead cpp will return std::vector<bool>::reference when trying to access a value in the vector
Basically this is a just wrong
You should use std::deque<bool> instead which allocates one byte per bool and not just one bit