Which Typesafe Enum in C++ Are You Using?

I'm currently playing around with the Boost.Enum proposal from the Boost Vault (filename enum_rev4.6.zip). Although it was never officially submitted for inclusion into Boost, it's useable as-is. (Documentation is lacking but is made up for by clear source code and good tests.)

Boost.Enum lets you declare an enum like this:

BOOST_ENUM_VALUES(Level, const char*,
    (Abort)("unrecoverable problem")
    (Error)("recoverable problem")
    (Alert)("unexpected behavior")
    (Info) ("expected behavior")
    (Trace)("normal flow of execution")
    (Debug)("detailed object state listings")
)

And have it automatically expand to this:

class Level : public boost::detail::enum_base<Level, string>
{
public:
    enum domain
    {
        Abort,
        Error,
        Alert,
        Info,
        Trace,
        Debug,
    };

    BOOST_STATIC_CONSTANT(index_type, size = 6);

    Level() {}
    Level(domain index) : boost::detail::enum_base<Level, string>(index) {}

    typedef boost::optional<Level> optional;
    static optional get_by_name(const char* str)
    {
        if(strcmp(str, "Abort") == 0) return optional(Abort);
        if(strcmp(str, "Error") == 0) return optional(Error);
        if(strcmp(str, "Alert") == 0) return optional(Alert);
        if(strcmp(str, "Info") == 0) return optional(Info);
        if(strcmp(str, "Trace") == 0) return optional(Trace);
        if(strcmp(str, "Debug") == 0) return optional(Debug);
        return optional();
    }

private:
    friend class boost::detail::enum_base<Level, string>;
    static const char* names(domain index)
    {
        switch(index)
        {
        case Abort: return "Abort";
        case Error: return "Error";
        case Alert: return "Alert";
        case Info: return "Info";
        case Trace: return "Trace";
        case Debug: return "Debug";
        default: return NULL;
        }
    }

    typedef boost::optional<value_type> optional_value;
    static optional_value values(domain index)
    {
        switch(index)
        {
        case Abort: return optional_value("unrecoverable problem");
        case Error: return optional_value("recoverable problem");
        case Alert: return optional_value("unexpected behavior");
        case Info: return optional_value("expected behavior");
        case Trace: return optional_value("normal flow of execution");
        case Debug: return optional_value("detailed object state listings");
        default: return optional_value();
        }
    }
};

It satisfies all five of the priorities which you list.

A nice compromise method is this:

struct Flintstones {
   enum E {
      Fred,
      Barney,
      Wilma
   };
};

Flintstones::E fred = Flintstones::Fred;
Flintstones::E barney = Flintstones::Barney;

It's not typesafe in the same sense that your version is, but the usage is nicer than standard enums, and you can still take advantage of integer conversion when you need it.

I use C++0x typesafe enums. I use some helper template/macros that provide the to/from string functionality.

enum class Result { Ok, Cancel};

I don't. Way too much overhead for little benefit. Also, being able to caste enumerations to different data types for serialization is a very handy tool. I have never seen an instance where a "Type safe" enumeration would be worth the overhead and complexity where C++ offers a good enough implementation already.

My take is that you're inventing a problem and then fitting a solution onto it. I see no need to do an elaborate framework for an enumeration of values. If you are dedicated to having your values only be members of a certain set, you could hack up a variant of a unique set datatype.

I think the Java enum would be a good model to follow. Essentially, the Java form would look like this:

public enum Result {
    OK("OK"), CANCEL("Cancel");

    private final String name;

    Result(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

What's interesting about the Java approach is that OK and CANCEL are immutable, singleton instances of Result (with the methods that you see). You cannot create any further instances of Result. Since they're singletons, you can compare by pointer/reference---very handy. :-)

ETA: In Java, instead of doing bitmasks by hand, instead you use an EnumSet to specify a bit set (it implements the Set interface, and works like sets---but implemented using bitmasks). Much more readable than hand-written bitmask manipulation!

I gave an answer to this here, on a different topic. It's a different style of approach which allows most of the same functionality without requiring modification to the original enum definition (and consequently allowing usage in cases where you don't define the enum). It also allows runtime range checking.

The downside of my approach is that it doesn't programmatically enforce the coupling between the enum and the helper class, so they have to be updated in parallel. It works for me, but YMMV.

Use boost::variant!

After trying a lot of the above ideas and finding them lacking I hit upon this simple approach:

#include <iostream>
#include <boost/variant.hpp>

struct A_t {};
static const A_t A = A_t();
template <typename T>
bool isA(const T & x) { if(boost::get<A_t>(&x)) return true; return false; }

struct B_t {};
static const B_t B = B_t();
template <typename T>
bool isB(const T & x) { if(boost::get<B_t>(&x)) return true; return false; }

struct C_t {};
static const C_t C = C_t();
template <typename T>
bool isC(const T & x) { if(boost::get<C_t>(&x)) return true; return false; }

typedef boost::variant<A_t, B_t> AB;
typedef boost::variant<B_t, C_t> BC;

void ab(const AB & e)
{
  if(isA(e))
    std::cerr << "A!" << std::endl;
  if(isB(e))
    std::cerr << "B!" << std::endl;
  // ERROR:
  // if(isC(e))
  //   std::cerr << "C!" << std::endl;

  // ERROR:
  // if(e == 0)
  //   std::cerr << "B!" << std::endl;
}

void bc(const BC & e)
{
  // ERROR:
  // if(isA(e))
  //   std::cerr << "A!" << std::endl;

  if(isB(e))
    std::cerr << "B!" << std::endl;
  if(isC(e))
    std::cerr << "C!" << std::endl;
}

int main() {
  AB a;
  a = A;
  AB b;
  b = B;
  ab(a);
  ab(b);
  ab(A);
  ab(B);
  // ab(C); // ERROR
  // bc(A); // ERROR
  bc(B);
  bc(C);
}

You can probably come up with a macro to generate the boilerplate. (Let me know if you do.)

Unlike other approaches this one is actually type-safe and works with old C++. You can even make cool types like boost::variant<int, A_t, B_t, boost::none>, for example, to represent a value that could be A, B, an integer or nothing which is almost Haskell98 levels of type safety.

Downsides to be aware of:

• at-least with old boost -- I'm on a system with boost 1.33 -- you are limited to 20 items in your variant; there is a work-around however
• affects compile time
• insane error messages -- but that's C++ for you

Update

Here, for your convenience is your typesafe-enum "library". Paste this header:

#ifndef _TYPESAFE_ENUMS_H
#define _TYPESAFE_ENUMS_H
#include <string>
#include <boost/variant.hpp>

#define ITEM(NAME, VAL) \
struct NAME##_t { \
  std::string toStr() const { return std::string( #NAME ); } \
  int toInt() const { return VAL; } \
}; \
static const NAME##_t NAME = NAME##_t(); \
template <typename T> \
bool is##NAME(const T & x) { if(boost::get<NAME##_t>(&x)) return true; return false; } \


class toStr_visitor: public boost::static_visitor<std::string> {
public:
  template<typename T>
  std::string operator()(const T & a) const {
    return a.toStr();
  }
};

template<BOOST_VARIANT_ENUM_PARAMS(typename T)>
inline static
std::string toStr(const boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> & a) {
  return boost::apply_visitor(toStr_visitor(), a);
}

class toInt_visitor: public boost::static_visitor<int> {
public:
  template<typename T>
  int operator()(const T & a) const {
    return a.toInt();
  }
};

template<BOOST_VARIANT_ENUM_PARAMS(typename T)>
inline static
int toInt(const boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> & a) {
  return boost::apply_visitor(toInt_visitor(), a);
}

#define ENUM(...) \
typedef boost::variant<__VA_ARGS__>
#endif

And use it like:

ITEM(A, 0);
ITEM(B, 1);
ITEM(C, 2);

ENUM(A_t, B_t) AB;
ENUM(B_t, C_t) BC;

Notice you have to say A_t instead of A in the ENUM macro which destroys some of the magic. Oh well. Also, notice there's now a toStr function and a toInt function to meet OPs requirement of simple conversion to strings and ints. The requirement I can't figure out is a way to iterate over the items. Let me know if you know how to write such a thing.

Not sure if this post is too late, but there's an article on GameDev.net which satisfies all but the 5th point (ability to iterate over enumerators): http://www.gamedev.net/reference/snippets/features/cppstringizing/

The method described by the article allows string conversion support for existing enumerations without changing their code. If you only want support for new enumerations though, I'd go with Boost.Enum (mentioned above).