std::bitset::count(3)

NAME
std::bitset::count - std::bitset::count

Synopsis
   std::size_t count() const; (noexcept since C++11)
                        (constexpr since C++23)
   Returns the number of bits that are set to true.

Parameters
   (none)

Return value
   Number of bits that are set to true.

Example
// Run this code
 #include <bitset>
 #include <iostream>
 int main()
 {
     std::bitset<8> b("00010010");
     std::cout << "Initial value: " << b << '\n';
     // Find the first unset bit
     std::size_t idx = 0;
     while (idx < b.size() && b.test(idx))
         ++idx;
     // Continue setting bits until half the bitset is filled
     while (idx < b.size() && b.count() < b.size() / 2)
     {
         b.set(idx);
         std::cout << "Setting bit " << idx << ": " << b << '\n';
         while (idx < b.size() && b.test(idx))
             ++idx;
     }
 }

Output:
 Initial value: 00010010
 Setting bit 0: 00010011
 Setting bit 2: 00010111

See also
   size returns the number of bits that the bitset holds
            (public member function)
   popcount counts the number of 1 bits in an unsigned integer
   (C++20) (function template)