enum class constructor c++ , how to pass specific value?

Question

I came from Java and here we have such option as set value to constuctor.

Example

enum TYPE
{
    AUTO(\"BMW\"),
    MOTOCYCLE(\"Kawasaki\");

    pr         


        

Answer 1

C++ ain't Java! Every language has its own techniques which are a good fit with the language. Don't try to mimic a perfectly fine construct of one language in the same (but broken) construct in a different language.

Here is how I would solve your issue in C++:

 // Define the actual enumeration
 enum class [[nodiscard]] Vehicle : unsigned char
 {
     CAR,
     MOTORCYCLE,
     SIZE [[maybe_unused]]
 };

// Convert your enumeration to a string (view)
#include <cassert>
#include <string_view>
[[nodiscard]] constexpr auto to_string(Vehicle v) noexcept -> std::string_view {
  assert(v != Vehicle::SIZE);
  switch (v) {
    case Vehicle::CAR:
      return "Car";
    case Vehicle::MOTORCYCLE:
      return "Motorcycle";
  }
}

To use it, you can do something like:

 for (unsigned char c = 0; c < static_cast<unsigned char>(Vehicle::SIZE); ++c)
        std::cout << to_string(static_cast<Vehicle>(c)) << std::endl;

Writing this every time is a bit cumbersome, however, you could write your own template class that helps with iterating over it. For example:

#include <type_traits>
// The generic stuff you only write once
// Assumes you don't assign any values to your enumeration by hand + it ends on
// 'SIZE' (unless a second argument was passed if a different name was used)
template <typename TEnumeration, TEnumeration TSize = TEnumeration::SIZE>
class [[nodiscard]] EnumRange final {
  using type = std::underlying_type_t<TEnumeration>;

 public:
  // The iterator that can be used to loop through all values
  //
  class [[nodiscard]] Iterator final {
    TEnumeration value{static_cast<TEnumeration>(0)};

   public:
    constexpr Iterator() noexcept = default;
    constexpr Iterator(TEnumeration e) noexcept : value{e} {}

    constexpr auto operator*() const noexcept -> TEnumeration { return value; }
    constexpr auto operator-> () const & noexcept -> const TEnumeration* {
      return &value;
    }
    constexpr auto operator++() & noexcept -> Iterator {
      value = static_cast<TEnumeration>(1 + static_cast<type>(value));
      return *this;
    }

    [[nodiscard]] constexpr auto operator==(Iterator i) -> bool { return i.value == value; }
    [[nodiscard]] constexpr auto operator!=(Iterator i) -> bool { return i.value != value; }
  };

  constexpr auto begin() const noexcept -> Iterator { return Iterator{}; }
  constexpr auto cbegin() const noexcept -> Iterator { return Iterator{}; }

  constexpr auto end() const noexcept -> Iterator { return Iterator{TSize}; }
  constexpr auto cend() const noexcept -> Iterator { return Iterator{TSize}; }

  [[nodiscard]] constexpr auto size() const noexcept -> type {
    return static_cast<type>(TSize);
  }
};

The usage:

#include <iostream>
int main(int, char**) {
  auto range = EnumRange<Vehicle>{};
  std::cout << static_cast<int>(range.size()) << std::endl;
  for (auto v : range) std::cout << to_string(v) << std::endl;
}

As you saw in the first test code, you can go from a numeric value to an enumeration by using static_cast. However, it assumes that you have some value that is valid for the enumeration. With the same assumptions of the range, we can write our own checked variant:

#include <stdexcept>
#include <type_traits>

template <typename TEnumeration, TEnumeration TSize = TEnumeration::SIZE>
[[nodiscard]] constexpr auto checked_enum_cast(
    std::underlying_type_t<TEnumeration> numeric_value) noexcept(false)
    -> TEnumeration {
        using type = std::underlying_type_t<TEnumeration>;
  if constexpr (std::is_signed_v<type>)
    if (numeric_value < 0) throw std::out_of_range{"Negative value"};

  if (numeric_value >= static_cast<type>(TSize)) throw std::out_of_range{"Value too large"};

  return static_cast<TEnumeration>(numeric_value);
}

To use this, you can write:

  try {
    std::cout << to_string(checked_enum_cast<Vehicle>(1)) << std::endl;
    std::cout << to_string(checked_enum_cast<Vehicle>(2)) << std::endl;
  } catch (const std::out_of_range& e) {
    std::cout << e.what() << std::endl;
  }

Note: If one would live in an exception-free world, one could return std::nullopt and change the return type to std::optional<TEnumeration> instead.

All code combined + execution at Compiler Explorer

Please note that the iterator can be refined, however, I ain't an expert in the details. (and for looping, it doesn't matter, if you ever want to use it for an algorithm it could)

Answer 2

In C++ a class has to be created:

class TYPE 
{
public:
    static const TYPE AUTO;
    static const TYPE MOTOCYCLE;

private:
    std::string mBrandName;

    TYPE(std::string iBrandName)
        : mBrandName(iBrandName)
    {}

    TYPE(const TYPE&) = default;
    TYPE(TYPE&&)      = default;
    TYPE& operator=(const TYPE&) = default;
    TYPE& operator=(TYPE&&) = default;
    ~TYPE() = default;

public:
    std::string getBrandName() { return mBrandName; }

    static TYPE getMotocycle() { return MOTOCYCLE; }
    static TYPE getAuto() { return AUTO; }
};

const TYPE TYPE::AUTO("BMW");
const TYPE TYPE::MOTOCYCLE("Kawasaki");

But this doesn't have the benefits of an enum (automatic numbering, ordering, conversions,...)