std::bitset::size
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | std::size_t size() const; | (until C++11) | |
| constexpr std::size_t size(); | (since C++11) (until C++14) | |
| constexpr std::size_t size() const; | (since C++14) | |
Returns the number of bits that the bitset can hold.
Parameters
(none)
Return value
number of bits that the bitset can hold, i.e. the template parameter N
Exceptions
| (none) | (until C++11) | 
| noexcept specification:   noexcept | (since C++11) | 
See also
| returns the number of bits set to true (public member function) |