Creating a string list and an enum list from a C++ macro

Question

In order to make my code shorter and easier to change I want to replace something like

enum{ E_AAA, E_BBB, E_CCC };
static const char *strings{\"AAA\", \"BBB         


        

Answer 1

I am a bit late to the party but here is another suggestion.
It creates a strongly typed enum class, say MyEnumName and a companion static helper class Enumator<MyEnumName>.
It's bigger than previous answers as it has more features, e.g. stream operators for conversion from/to string.
Note that it relies on c++ 14 standard due to the use of index sequence.

Usage:

/* One line definition - no redundant info */
ENUM_DEFINE(WeekDay /*first item is enum name*/, 
    Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday);

/* works seemlessly with streams (good for logging) */
auto dayOne = WeekDay::Sunday;
std::cout << "day of week is: " << day_of_week;

/* explicit construction from string using WeekDay_enum companion class*/
auto dayTwo = Enumator<WeekDay>::fromString("Tuesday");


/*Iterate over all enum values using Enumator<WeekDay> companion class*/
std::cout << "Days of the week are:\n"
for (auto enumVal : Enumator<WeekDay>::getValues()) {
    std::cout << enumVal << "\n";
}

Source:

    #include <array>
    #include <string>
    #include <sstream>
    #include <stdexcept>

template<typename E>
using isEnum = typename std::enable_if<std::is_enum<E>::value>::type;

template<typename E, typename = isEnum<E>>
constexpr static int enumSize() {
    return 0;
}

template<typename E, typename = isEnum<E>>
inline static std::string getEnumStringValues() {
    return "";
}


/*Enum companion class to hold the methods that can't be declared in an enum*/
template<typename EnumType, isEnum<EnumType>* = nullptr>
class Enumator
{
    Enumator() = delete; /* prevents instantiation */

public:

    constexpr static int size() {
        return enumSize<EnumType>();
    }
    /* list of all enum values a string */
    static auto const& getValuesStr()
    {
        static std::array<std::string, size()> values;
        if (values[0].empty()) {
            std::string valuesStr = getEnumStringValues<EnumType>();
            std::stringstream ss(valuesStr);
            for (auto& value : values) {
                std::getline(ss, value, ',');                   
            }
        }
        return values;
    };

    /* list of all enum values */
    static auto const& getValues()
    {
        static std::array<EnumType, size()> values{ make_array(std::make_index_sequence<size()>()) };
        return values;
    };

    /* To/from string conversion */
    constexpr static std::string const& toString(EnumType arg) {
        return getValuesStr()[static_cast<unsigned>(arg)];
    }

    static EnumType fromString(std::string const& val)
    {
        /* Attempt at converting from string value */
        auto const& strValues = getValuesStr();

        for (unsigned int i = 0; i < strValues.size(); i++)
        { 
            if (val == strValues[i])
            {
                return static_cast<EnumType>(i);
            }
        }
        throw std::runtime_error("No matching enum value found for token: " + val);
    }

private:
    /* Helper method to initialize array of enum values */
    template<std::size_t...Idx>
    static auto make_array(std::index_sequence<Idx...>)
    {
        return std::array<EnumType, size()>{{static_cast<EnumType>(Idx)...}};
    }
};

template<typename EnumType, isEnum<EnumType>* = nullptr>
inline std::istream& operator>> (std::istream& input, EnumType& arg)
{
    std::string val;
    input >> val;
    arg = Enumator<EnumType>::fromString(val);
    return input;
}

template<typename EnumType, isEnum<EnumType>* = nullptr>
inline std::ostream& operator<< (std::ostream& output, const EnumType& arg)
{
    return output << Enumator<EnumType>::toString(arg);
}

#define ENUM_DEFINE(EnumName,...)\
    \
    enum class EnumName;\
    \
    template<>\
    constexpr int enumSize<EnumName>() {\
        /*Trick to get the number of enum members:*/\
        /*dump all the enum values in an array and compute its size */\
        enum EnumName { __VA_ARGS__ }; \
        EnumName enumArray[]{ __VA_ARGS__ }; \
        return sizeof(enumArray) / sizeof(enumArray[0]); \
    }\
    \
    template<>\
    inline std::string getEnumStringValues<EnumName>() { return #__VA_ARGS__; }\
    \
    enum class EnumName : int { __VA_ARGS__ }

Answer 2

For a simple solution, I'd recommend something like X-Macros.

For a more complex solution that adds several other features (like range checking, enhanced type safety, optional associated data, etc.), there's a proposed (but never finalized) Boost.Enum library.

Answer 3

One way to do this is with X-Macros, which are basically a way to define a macro which is then used for generating more complex structures than a simple macro easily allows. Here is an example of doing exactly what you are asking.

Answer 4

You can do it with a bit of macro magic:

#define FRUITS \
    etype(Unknown), \
    etype(Apple),   \
    etype(Orange),  \
    etype(Banana),  \
    etype(Apricot), \
    etype(Mango)

#define etype(x) F_##x

typedef enum { FRUITS } Fruit;

#undef etype
#define etype(x) #x

static const char *strFruit[] = { FRUITS };

Here is a test program:

#include <iostream>
#include <exception>
#include <vector>

#define FRUITS \
    etype(Unknown), \
    etype(Apple),   \
    etype(Orange),  \
    etype(Banana),  \
    etype(Apricot), \
    etype(Mango)

#define etype(x) F_##x

typedef enum { FRUITS } Fruit;

#undef etype
#define etype(x) #x

static const char *strFruit[] = { FRUITS };

const char *enum2str (Fruit f)
{
    return strFruit[static_cast<int>(f)];
}

Fruit str2enum (const char *f)
{
    const int n = sizeof(strFruit) / sizeof(strFruit[0]);
    for (int i = 0; i < n; ++i)
    {
        if (strcmp(strFruit[i], f) == 0)
            return (Fruit) i;
    }
    return F_Unknown;
}

int main (int argc, char *argv[])
{
    std::cout << "I like " << enum2str(F_Mango) << std::endl;
    std::cout << "I do not like " << enum2str(F_Banana) << std::endl;
    std::vector<char *> v;
    v.push_back("Apple");
    v.push_back("Mango");
    v.push_back("Tomato");
    for (int i = 0; i < v.size(); ++i)
    {
        const Fruit f = str2enum(v[i]);
        if (f == F_Unknown)
            std::cout << "Is " << v[i] << " a fruit?" << std::endl;
        else
            std::cout << v[i] << " is a fruit" << std::endl;
    }
    return 0;
}

It outputs:

I like Mango
I do not like Banana
Apple is a fruit
Mango is a fruit
Is Tomato a fruit?

Answer 5

Here is a solution with Boost.Preprocessor:

#include <boost/preprocessor.hpp>

#define DEFINE_ENUM_DECL_VAL(r, name, val) BOOST_PP_CAT(name, BOOST_PP_CAT(_, val))
#define DEFINE_ENUM_VAL_STR(r, name, val) BOOST_PP_STRINGIZE(val)
#define DEFINE_ENUM(name, val_seq)                                                 \
  enum name {                                                                      \
    BOOST_PP_SEQ_ENUM(BOOST_PP_SEQ_TRANSFORM(DEFINE_ENUM_DECL_VAL, name, val_seq)) \
  };                                                                               \
  static const char* BOOST_PP_CAT(name, _strings[] = ) {                           \
    BOOST_PP_SEQ_ENUM(BOOST_PP_SEQ_TRANSFORM(DEFINE_ENUM_VAL_STR, name, val_seq)) \
  };

DEFINE_ENUM(E, (AAA)(BBB)(CCC))

(AAA)(BBB)(CCC) is a Boost.Preprocessor sequence of tree elements AAA, BBB and CCC; the macro append the enum name to it's modalities:

enum E { E_AAA, E_BBB, E_CCC };
static const char* E_strings[] = { "AAA", "BBB", "CCC" };

Answer 6

One way to handle this is to define a list macro, i.e. something that expands to another macro that is left for the user to define. For example:

#define MY_LIST MY_ENTRY(AAA) MY_ENTRY(BBB) MY_ENTRY(CCC)

To define the enum:

#define MY_ENTRY(x) E_##x,
enum name
{
  MY_LIST
  NUMBER_OF_ELEMENTS    /* Needed to eat trailing comma (not needed in C99, but in C++) */
};
#undef MY_ENTRY

To define the string:

#define MY_ENTRY(x) #x,
static const char *strings[] = { MY_LIST };
#undef MY_ENTRY

Personally, I find this much easier to work with than the X macro, as this does not rely in include-file magic.