enum to string in modern C++11 / C++14 / C++17 and

Question

Contrary to all other similar questions, this question is about using the new C++ features.

• 2008 c Is there a simple way to convert C++ enum to string?
• 2008 c Easy way to use variables of enum types as string in C?
• 2008 c++ How to easily map c++ enums to strings
• 2008 c++ Making something both a C identifier and a string?
• 2008 c++ Is there a simple script to convert C++ enum to string?
• 2009 c++ How to use enums as flags in C++?
• 2011 c++ How to convert an enum type variable to a string?
• 2011 c++ Enum to String C++
• 2011 c++ How to convert an enum type variable to a string?
• 2012 c How to convert enum names to string in c
• 2013 c Stringifying an conditionally compiled enum in C

After reading many answers, I did not yet find any:

• Elegant way using C++11, C++14 or C++17 new features
• Or something ready-to-use in Boost
• Else something planned for C++20

Example

An example is often better than a long explanation.
You can compile and run this snippet on Coliru.
^{(Another former example is also available)}

#include <map>
#include <iostream>

struct MyClass
{
    enum class MyEnum : char {
        AAA = -8,
        BBB = '8',
        CCC = AAA + BBB
    };
};

// Replace magic() by some faster compile-time generated code
// (you're allowed to replace the return type with std::string
// if that's easier for you)
const char* magic (MyClass::MyEnum e)
{
    const std::map<MyClass::MyEnum,const char*> MyEnumStrings {
        { MyClass::MyEnum::AAA, "MyClass::MyEnum::AAA" },
        { MyClass::MyEnum::BBB, "MyClass::MyEnum::BBB" },
        { MyClass::MyEnum::CCC, "MyClass::MyEnum::CCC" }
    };
    auto   it  = MyEnumStrings.find(e);
    return it == MyEnumStrings.end() ? "Out of range" : it->second;
}

int main()
{
   std::cout << magic(MyClass::MyEnum::AAA) <<'\n';
   std::cout << magic(MyClass::MyEnum::BBB) <<'\n';
   std::cout << magic(MyClass::MyEnum::CCC) <<'\n';
}

Constraints

• Please no invaluable duplication of other answers or basic link.
• Please avoid bloat macro-based answer, or try to reduce the #define overhead as minimum as possible.
• Please no manual enum -> string mapping.

Nice to have

• Support enum values starting from a number different from zero
• Support negative enum values
• Support fragmented enum values
• Support class enum (C++11)
• Support class enum : <type> having any allowed <type> (C++11)
• Compile-time (not run-time) conversions to a string,
  or at least fast execution at run-time (e.g. std::map is not a great idea...)
• constexpr (C++11, relaxed in C++14)
• noexcept (C++11)
• snippet C++14/C++17 friendly
• C++ State of the art

One possible idea could be using the C++ compiler capabilities to generate C++ code at compilation-time using meta-programming tricks based on variadic template class and constexpr functions...

Answer 1

Back in 2011 I spent a weekend fine-tuning a macro-based solution and ended up never using it.

My current procedure is to start Vim, copy the enumerators in an empty switch body, start a new macro, transform the first enumerator into a case statement, move the cursor to the beginning of the next line, stop the macro and generate the remaining case statements by running the macro on the other enumerators.

Vim macros are more fun than C++ macros.

Real-life example:

enum class EtherType : uint16_t
{
    ARP   = 0x0806,
    IPv4  = 0x0800,
    VLAN  = 0x8100,
    IPv6  = 0x86DD
};

I will create this:

std::ostream& operator<< (std::ostream& os, EtherType ethertype)
{
    switch (ethertype)
    {
        case EtherType::ARP : return os << "ARP" ;
        case EtherType::IPv4: return os << "IPv4";
        case EtherType::VLAN: return os << "VLAN";
        case EtherType::IPv6: return os << "IPv6";
        // omit default case to trigger compiler warning for missing cases
    };
    return os << static_cast<std::uint16_t>(ethertype);
}

And that's how I get by.

Native support for enum stringification would be much better though. I'm very interested to see the results of the reflection workgroup in C++17.

An alternative way to do it was posted by @sehe in the comments.

Answer 2

my solution is without macro usage.

advantages:

• you see exactly what you do
• access is with hash maps, so good for many valued enums
• no need to consider order or non-consecutive values
• both enum to string and string to enum translation, while added enum value must be added in one additional place only

disadvantages:

• you need to replicate all the enums values as text
• access in hash map must consider string case
• maintenance if adding values is painful - must add in both enum and direct translate map

so... until the day that C++ implements the C# Enum.Parse functionality, I will be stuck with this:

            #include <unordered_map>

            enum class Language
            { unknown, 
                Chinese, 
                English, 
                French, 
                German
                // etc etc
            };

            class Enumerations
            {
            public:
                static void fnInit(void);

                static std::unordered_map <std::wstring, Language> m_Language;
                static std::unordered_map <Language, std::wstring> m_invLanguage;

            private:
                static void fnClear();
                static void fnSetValues(void);
                static void fnInvertValues(void);

                static bool m_init_done;
            };

            std::unordered_map <std::wstring, Language> Enumerations::m_Language = std::unordered_map <std::wstring, Language>();
            std::unordered_map <Language, std::wstring> Enumerations::m_invLanguage = std::unordered_map <Language, std::wstring>();

            void Enumerations::fnInit()
            {
                fnClear();
                fnSetValues();
                fnInvertValues();
            }

            void Enumerations::fnClear()
            {
                m_Language.clear();
                m_invLanguage.clear();
            }

            void Enumerations::fnSetValues(void)
            {   
                m_Language[L"unknown"] = Language::unknown;
                m_Language[L"Chinese"] = Language::Chinese;
                m_Language[L"English"] = Language::English;
                m_Language[L"French"] = Language::French;
                m_Language[L"German"] = Language::German;
                // and more etc etc
            }

            void Enumerations::fnInvertValues(void)
            {
                for (auto it = m_Language.begin(); it != m_Language.end(); it++)
                {
                    m_invLanguage[it->second] = it->first;
                }
            }

            // usage -
            //Language aLanguage = Language::English;
            //wstring sLanguage = Enumerations::m_invLanguage[aLanguage];

            //wstring sLanguage = L"French" ;
            //Language aLanguage = Enumerations::m_Language[sLanguage];

Answer 3

The following solution is based on a std::array<std::string,N> for a given enum.

For enum to std::string conversion we can just cast the enum to size_t and lookup the string from the array. The operation is O(1) and requires no heap allocation.

#include <boost/preprocessor/seq/transform.hpp>
#include <boost/preprocessor/seq/enum.hpp>
#include <boost/preprocessor/stringize.hpp>

#include <string>
#include <array>
#include <iostream>

#define STRINGIZE(s, data, elem) BOOST_PP_STRINGIZE(elem)

// ENUM
// ============================================================================
#define ENUM(X, SEQ) \
struct X {   \
    enum Enum {BOOST_PP_SEQ_ENUM(SEQ)}; \
    static const std::array<std::string,BOOST_PP_SEQ_SIZE(SEQ)> array_of_strings() { \
        return {{BOOST_PP_SEQ_ENUM(BOOST_PP_SEQ_TRANSFORM(STRINGIZE, 0, SEQ))}}; \
    } \
    static std::string to_string(Enum e) { \
        auto a = array_of_strings(); \
        return a[static_cast<size_t>(e)]; \
    } \
}

For std::string to enum conversion we would have to make a linear search over the array and cast the array index to enum.

Try it here with usage examples: http://coliru.stacked-crooked.com/a/e4212f93bee65076

Edit: Reworked my solution so the custom Enum can be used inside a class.

Answer 4

I wrote a library for solving this problem, everything happens in compiling time, except for getting the message.

Usage:

Use macro DEF_MSG to define a macro and message pair:

DEF_MSG(CODE_OK,   "OK!")
DEF_MSG(CODE_FAIL, "Fail!")

CODE_OK is the macro to use, and "OK!" is the corresponding message.

Use get_message() or just gm() to get the message:

get_message(CODE_FAIL);  // will return "Fail!"
gm(CODE_FAIL);           // works exactly the same as above

Use MSG_NUM to find out how many macros have been defined. This will automatically increse, you don't need to do anything.

Predefined messages:

MSG_OK:     OK
MSG_BOTTOM: Message bottom

Project: libcodemsg

---

The library doesn't create extra data. Everything happens in compiling time. In message_def.h, it generates an enum called MSG_CODE; in message_def.c, it generates a variable holds all the strings in static const char* _g_messages[].

In such case, the library is limited to create one enum only. This is ideal for return values, for example:

MSG_CODE foo(void) {
    return MSG_OK; // or something else
}

MSG_CODE ret = foo();

if (MSG_OK != ret) {
    printf("%s\n", gm(ret););
}

Another thing I like this design is you can manage message definitions in different files.

---

I found the solution to this question looks much better.

Answer 5

This gist provides a simple mapping based on C++ variadic templates.

This is a C++17-simplified version of the type-based map from the gist:

#include <cstring> // http://stackoverflow.com/q/24520781

template<typename KeyValue, typename ... RestOfKeyValues>
struct map {
  static constexpr typename KeyValue::key_t get(const char* val) noexcept {
    if constexpr (sizeof...(RestOfKeyValues)==0)  // C++17 if constexpr
      return KeyValue::key; // Returns last element
    else {
      static_assert(KeyValue::val != nullptr,
                  "Only last element may have null name");
      return strcmp(val, KeyValue::val()) 
            ? map<RestOfKeyValues...>::get(val) : KeyValue::key;
    }
  }
  static constexpr const char* get(typename KeyValue::key_t key) noexcept {
    if constexpr (sizeof...(RestOfKeyValues)==0)
      return (KeyValue::val != nullptr) && (key == KeyValue::key)
            ? KeyValue::val() : "";
    else
      return (key == KeyValue::key) 
            ? KeyValue::val() : map<RestOfKeyValues...>::get(key);
  }
};

template<typename Enum, typename ... KeyValues>
class names {
  typedef map<KeyValues...> Map;
public:
  static constexpr Enum get(const char* nam) noexcept {
    return Map::get(nam);
  }
  static constexpr const char* get(Enum key) noexcept {
    return Map::get(key);
  }
};

An example usage:

enum class fasion {
    fancy,
    classic,
    sporty,
    emo,
    __last__ = emo,
    __unknown__ = -1
};

#define NAME(s) static inline constexpr const char* s() noexcept {return #s;}
namespace name {
    NAME(fancy)
    NAME(classic)
    NAME(sporty)
    NAME(emo)
}

template<auto K, const char* (*V)()>  // C++17 template<auto>
struct _ {
    typedef decltype(K) key_t;
    typedef decltype(V) name_t;
    static constexpr key_t  key = K; // enum id value
    static constexpr name_t val = V; // enum id name
};

typedef names<fasion,
    _<fasion::fancy, name::fancy>,
    _<fasion::classic, name::classic>,
    _<fasion::sporty, name::sporty>,
    _<fasion::emo, name::emo>,
    _<fasion::__unknown__, nullptr>
> fasion_names;

The map<KeyValues...> can be used in both directions:

• fasion_names::get(fasion::emo)
• fasion_names::get("emo")

This example is available on godbolt.org

int main ()
{
  constexpr auto str = fasion_names::get(fasion::emo);
  constexpr auto fsn = fasion_names::get(str);
  return (int) fsn;
}

Result from gcc-7 -std=c++1z -Ofast -S

main:
        mov     eax, 3
        ret

Answer 6

Well, yet another option. A typical use case is where you need constants for the HTTP verbs as well as using its string version values.

The example:

int main () {

  VERB a = VERB::GET;
  VERB b = VERB::GET;
  VERB c = VERB::POST;
  VERB d = VERB::PUT;
  VERB e = VERB::DELETE;


  std::cout << a.toString() << std::endl;

  std::cout << a << std::endl;

  if ( a == VERB::GET ) {
    std::cout << "yes" << std::endl;
  }

  if ( a == b ) {
    std::cout << "yes" << std::endl;
  }

  if ( a != c ) {
    std::cout << "no" << std::endl;
  }

}

The VERB class:

// -----------------------------------------------------------
// -----------------------------------------------------------
class VERB {

private:

  // private constants
  enum Verb {GET_=0, POST_, PUT_, DELETE_};

  // private string values
  static const std::string theStrings[];

  // private value
  const Verb value;
  const std::string text;

  // private constructor
  VERB (Verb v) :
  value(v), text (theStrings[v])
  {
    // std::cout << " constructor \n";
  }

public:

  operator const char * ()  const { return text.c_str(); }

  operator const std::string ()  const { return text; }

  const std::string toString () const { return text; }

  bool operator == (const VERB & other) const { return (*this).value == other.value; }

  bool operator != (const VERB & other) const { return ! ( (*this) == other); }

  // ---

  static const VERB GET;
  static const VERB POST;
  static const VERB PUT;
  static const VERB DELETE;

};

const std::string VERB::theStrings[] = {"GET", "POST", "PUT", "DELETE"};

const VERB VERB::GET = VERB ( VERB::Verb::GET_ );
const VERB VERB::POST = VERB ( VERB::Verb::POST_ );
const VERB VERB::PUT = VERB ( VERB::Verb::PUT_ );
const VERB VERB::DELETE = VERB ( VERB::Verb::DELETE_ );
// end of file