`bit::vector` — Storage Capacity

How many elements can a bit-vector store in its current state?

1constexpr std::size_t capacity() const;
2constexpr std::size_t unused() const;

1: Returns the number of elements the bit-vector can store in its current state.
2: Returns the spare capacity in the bit-vector in its current state, i.e., capacity() - size().

We may not be using all the storage for some bit-vectors. For example, if we construct a small bit::vector with, say, eight elements and use the default Block of uint64_t, we will have at least a single storage block, so a capacity of 64. Thus, there are 56 spare slots — we can append 56 more elements to the vector before allocating more storage.

Example

#include <bit/bit.h>
int main()
{
    auto v = bit::vector<>::checker_board(8);
    std::cout << "bit::vector "     << v            << ": ";
    std::cout << "size "            << v.size()     << ", ";
    std::cout << "capacity "        << v.capacity() << ", ";
    std::cout << "unused capacity " << v.unused()   << ".\n";
}

Output

bit::vector [0 1 0 1 0 1 0 1]: size 8, capacity 64, unused capacity 56

See Also