Rust's Option type and Result type in C++ using tagged unions

option-and-result.cpp

//clang 3.8.0

#include <iostream>

// Option type and Result type implementations (tagged unions).

enum class OptionType { Some, None };

template<typename T>
struct Option
{
    OptionType type;
    union
    {
        struct { T value; }; // Some
        struct {};           // None
    };
};

enum class ResultType { Ok, Error };

template<typename T, typename E>
struct Result
{
    ResultType type;
    union
    {
        struct { T value; }; // Ok
        struct { E error; }; // Error
    };
};

// Methods that use Option type and Result type.

enum class ErrorCode { DIVISION_BY_ZERO };

template<typename T>
Option<T> option_div(T numerator, T denominator)
{
    if (denominator == 0)
    {
        return Option<T> { OptionType::None };
    }
    return Option<T> { OptionType::Some, .value = (numerator / denominator) };
}

template<typename T>
Result<T, ErrorCode> result_div(T numerator, T denominator)
{
    if (denominator == 0)
    {
        return Result<T, ErrorCode> { ResultType::Error, .error = ErrorCode::DIVISION_BY_ZERO };
    }
    return Result<T, ErrorCode> { ResultType::Ok, .value = (numerator / denominator) };
}

// Helpers.

template<typename T>
void print(Option<T> option)
{
    switch (option.type)
    {
        case OptionType::Some:
            std::cout << "Some: " << option.value << std::endl;
            break;
        case OptionType::None:
            std::cout << "None" << std::endl;
            break;
        default:
            break;
    }
}

template<typename T, typename E>
void print(Result<T, E> result)
{
    switch (result.type)
    {
        case ResultType::Ok:
            std::cout << "Ok: " << result.value << std::endl;
            break;
        case ResultType::Error:
            switch (result.error)
            {
                case ErrorCode::DIVISION_BY_ZERO:
                    std::cout << "Error: Division by Zero" << std::endl;
                    break;
                default:
                    std::cout << "Error" << std::endl;
                    break;
            }
            break;
        default:
            break;
    }
}

// Examples.

int main()
{
    std::cout << "Option type examples" << std::endl;
    print(option_div(4, 2));
    print(option_div(4, 1));
    print(option_div(4, 0));

    std::cout << "Result type examples" << std::endl;
    print(result_div(4, 2));
    print(result_div(4, 1));
    print(result_div(4, 0));
}