Fast and maintainable way of implementing string to integer lookup table

Question

I can find many hits on the question on how to map an integer to a string constant, with the obvious solution

char const* strings[] = {"Foo", "Bar", ...};

Now, suppose I want the inverse: I have the string "Bar", and want 1. My strings are up to 4 characters and ascii null is not a valid value. There are 64 integers to map values to. Do I have to write a long if-else construct with string comparison or is there something better.

To clarify, I prefer the solution to not require runtime initialization, which as of C++17, makes ti impossible to use std::map or std::unordered_map.

Answer 1

My strings are up to 4 characters and ascii null is not a valid value.

With this condition, you can convert your strings into an integer, and use your favourite compile-time integer-to-integer map.

For example, with a switch:

#include <cstddef>
#include <cstdint>

namespace detail {
    constexpr std::uint64_t string_as_int(const char* string) noexcept {
        std::uint64_t result = 0;
        std::uint64_t i = 0;
        for (; i < 4 && *string; ++string, ++i) {
            result |= static_cast<std::uint64_t>(static_cast<unsigned char>(*string)) << (i * 8u);
        }
        return result;
    }

    constexpr std::uint64_t operator ""_h(const char* string, std::size_t) noexcept {
        return string_as_int(string);
    }
}

constexpr int lookup(const char* string) {
    using detail::operator ""_h;
    switch (detail::string_as_int(string)) {
        case "Foo"_h: return 1;
        case "Bar"_h: return 2;
        default: return 0;
    }
}