std::bitset<N>::size - cppreference.com

From cppreference.com

std::size_t size() const;

(noexcept since C++11)
(constexpr since C++11)

Returns the number of bits that the bitset holds.

Parameters

(none)

Return value

number of bits that the bitset holds, i.e. the template parameter N.

Example

#include <bitset>
#include <iostream>

int main()
{
    std::cout << std::bitset<0x400>().size() << '\n';
}

Output:

See also

returns the number of bits set to true
(public member function) [edit]