SwitchCString.cpp

// Question:
// @see: https://stackoverflow.com/questions/650162/why-the-switch-statement-cannot-be-applied-on-strings
//
// Solution:
// @see: https://blog.csdn.net/yozidream/article/details/22789147

#include <iostream>

namespace SimpleHash
{
    using hash_t = std::uint64_t;

    constexpr static hash_t prime{ 0x100000001B3ull };
    constexpr static hash_t basis{ 0xCBF29CE484222325ull };

    // C++11 constexpr functions use recursion rather than iteration
    // (C++14 constexpr functions may use local variables and loops)
    //
    // The parameter cannot have non-literal-type,
    // which means `std::string` cannot be
    // declared as the parameter.
    //
    // @see: https://en.cppreference.com/w/cpp/language/constexpr
    constexpr hash_t hash(const char* str)
    {
        hash_t ret{ basis };
        while (*str)
        {
            ret ^= *str;
            ret *= prime;
            str++;
        }
        return ret;
    }

    constexpr hash_t operator "" h(const char* s, size_t)
    {
        return hash(s);
    }
};

void TestSwitch(const char* s);

int main()
{
    std::string messages[] = {
        "Alpha", "Nothing", "Beta", "Now"
    };
    for (const std::string& message : messages)
    {
        TestSwitch(message.c_str());
    }
    // Expected output:
    // Alpha
    // Default
    // Beta
    // Default
    return 0;
}

void TestSwitch(const char* s)
{
    using namespace SimpleHash; // !!! IMPORTANT !!!

    switch (hash(s))
    {
    case "Alpha"h:
        std::cout << "Alpha" << std::endl;
        break;
    case "Beta"h:
        std::cout << "Beta" << std::endl;
        break;
    default:
        std::cout << "Default" << std::endl;
        break;
    }
}