Is there a built in function for std::string in C++ to compare two strings alphabetically when either string can be uppercase or lowercase?

Question

I know for C++ that basic comparative operators can accomplish the task if both words are either entirely lower or entirely upper case. I have an array of strings and letter

Answer 1

Yes there is a case insensitive way to compare strings in C++. The key is that std::string is a template:

template ,
          class Allocator = allocator>
class basic_string;

The traits here control how the charT's relate to each other. For normal std::string, they do what you'd expect, but we can just write our own traits that are case insensitive:

struct case_insensitive_traits
: char_traits
{
    static bool eq(char a, char b) { return tolower(a) == tolower(b); }
    static bool ne(char a, char b) { return !eq(a, b); }
    static bool lt(char a, char b) { return tolower(a) < tolower(b); }
    static bool gt(char a, char b) { return tolower(a) > tolower(b); }

    static int compare(const char* a, const char* b, size_t n)
    {
        for (size_t i = 0; i < n; ++i) {
            int delta = tolower(a[i]) - tolower(b[i]);
            if (delta != 0) return delta;
        }
        return 0;
    }

    static const char* find(const char* s, size_t n, char c)
    {
        c = tolower(c);
        for (size_t i = 0; i < n; ++i, ++s) {
            if (tolower(*s) == c) return s;
        }
        return nullptr;
    }
};

With that:

using case_insensitive_string = std::basic_string;

case_insensitive_string a{"hello"};
case_insensitive_string b{"hElLo"};

assert(a == b);