#include <string>
#include <iostream>
#include <cctype>
 
struct ci_char_traits : public std::char_traits<char> {
    static char to_upper(char ch) {
        return std::toupper((unsigned char) ch);
    }
    static bool eq(char c1, char c2) {
         return to_upper(c1) == to_upper(c2);
     }
    static bool lt(char c1, char c2) {
         return to_upper(c1) <  to_upper(c2);
    }
    static int compare(const char* s1, const char* s2, size_t n) {
        while ( n-- != 0 ) {
            if ( to_upper(*s1) < to_upper(*s2) ) return -1;
            if ( to_upper(*s1) > to_upper(*s2) ) return 1;
            ++s1; ++s2;
        }
        return 0;
    }
    static const char* find(const char* s, int n, char a) {
        auto const ua (to_upper(a));
        while ( n-- != 0 ) 
        {
            if (to_upper(*s) == ua)
                return s;
            s++;
        }
        return nullptr;
    }
};
 
using ci_string = std::basic_string<char, ci_char_traits>;
 
std::ostream& operator<<(std::ostream& os, const ci_string& str) {
    return os.write(str.data(), str.size());
}
 
int main()
{
    ci_string s1 = "Hello";
    ci_string s2 = "heLLo";
    if (s1 == s2)
        std::cout << s1 << " and " << s2 << " are equal\n";
}

Hello and heLLo are equal