`bit::vector` — Change the Capacity

Potentially change the vector::capacity of a bit-vector.

1constexpr bit::vector &reserve(std::size_t n);
2constexpr bit::vector &shrink_to_fit();

1: Increases the bit-vector’s vector::capacity to hold n elements.
Does nothing if n elements fit inside the current capacity.
2: This is a request to minimize the unused/excess vector::capacity. May do nothing.

The idea is to make it as efficient as possible to append a (known) large number of elements to a bit-vector by allocating the needed storage up-front rather than in pieces.

These methods return *this, so you can chain them with other calls.

These methods do not change a bit-vector’s size(). No elements are added or deleted.

If the capacity changes, all the old values remain unchanged. However, any references are then invalidated.

Example

#include <bit/bit.h>
int main()
{
    auto v = bit::vector<>::checker_board(8);
1    std::cout << v << ": size " << v.size() << ", capacity " << v.capacity() << '\n';
2    v.reserve(99);
3    std::cout << v << ": size " << v.size() << ", capacity " << v.capacity() << '\n';
4    v.shrink_to_fit();
    std::cout <<  v << ": size " << v.size() << ", capacity " << v.capacity() << '\n';
}

1: We’re using the default 64-bit blocks, so v can hold 64 elements (though it only has eight at present).
2: Get v “ready” to hold 99 elements.
3: As the blocks are all 64-bits, we need two for those 99 elements; therefore, the capacity increases to 128.
4: We changed our minds and want to shrink v to a minimum size. Note that the elements in v never changed!

Output

[1 0 1 0 1 0 1 0]: size 8, capacity 64
[1 0 1 0 1 0 1 0]: size 8, capacity 128
[1 0 1 0 1 0 1 0]: size 8, capacity 64

See Also