Mission Impossible / How to create datatypes which cannot contain invalid state

adrianimboden

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01 Introduction

Introduction

02 Enums

Enums - Overview Enums - Hands On Enums - Example Solution

03 Optional

Optional - Overview Optional - Hands On Optional - Example Solution

04 Variant

Variant - Overview Variant - Hands On Variant - Example Solution

05 Final

Final - Hands On Final - Example Solution

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Possible solution

// {
#include <iostream>
#include <variant>
// likely part of C++20
template <class... Ts>
struct overloaded : Ts... {
    using Ts::operator()...;
};
template <class... Ts>
overloaded(Ts...)->overloaded<Ts...>;
// }
enum class ErrorType {
    OutOfRangeForCheck,
    Timeout,
};
struct ErrorInfo {
    ErrorType   type;
    std::string detailInfo;
};
using Quality = std::variant<uint32_t, ErrorInfo>;
Quality check_quality(uint8_t production_result) {
    if (production_result < 10 || production_result > 20) {
        return ErrorInfo{ErrorType::OutOfRangeForCheck, "must be x >= 10 and x <= 20"};
    }
    return uint32_t{production_result} * 2;
}
void print_quality(Quality quality) {
    std::visit(
        overloaded{
            [](ErrorInfo error) { std::cout << "Error: " << error.detailInfo << "\n"; },
            [](uint32_t quality) { std::cout << "Quality: " << quality << "\n"; },
        },
        quality);
}
void do_one_production_cycle() {
    print_quality(check_quality(3));
    print_quality(check_quality(12));
    print_quality(check_quality(17));
}