Example for https://essential-cpp.programming-books.io/user-defined-literals-d3696e4f87894401af6c2f073d356228 (made with https://codeeval.dev)

main.cc

#include <iostream>

template< char FIRST, char... REST > struct binary
{
    static_assert( FIRST == '0' || FIRST == '1', "invalid binary digit" ) ;
    enum { value = ( ( FIRST - '0' ) << sizeof...(REST) ) + binary<REST...>::value  } ;
};

template<> struct binary<'0'> { enum { value = 0 } ; };
template<> struct binary<'1'> { enum { value = 1 } ; };

// raw literal operator
template< char... LITERAL > inline
constexpr unsigned int operator "" _b() { return binary<LITERAL...>::value ; }

// raw literal operator
template< char... LITERAL > inline
constexpr unsigned int operator "" _B() { return binary<LITERAL...>::value ; }

int main()
{
    // 10101 in binary is 21
    std::cout << 10101_B << "\n";
    // 011011000111 in binary is 1735
    std::cout << 011011000111_b  << '\n';
}