String to enum in C++

Question

Is there a way to associate a string from a text file with an enum value?

The problem is: I have a few enum values stored as string in a text file which I read on the fly on meeting some condition... Now I want to assign the read value to an enum.

What is the most effective way to do so? It doesn't need to be the simplest approach.

Answer 1

You can set up a map that you can use over and over:

template <typename T>
class EnumParser
{
    map <string, T> enumMap;
public:
    EnumParser(){};

    T ParseSomeEnum(const string &value)
    { 
        map <string, T>::const_iterator iValue = enumMap.find(value);
        if (iValue  == enumMap.end())
            throw runtime_error("");
        return iValue->second;
    }
};

enum SomeEnum
{
    Value1,
    Value2
};
EnumParser<SomeEnum>::EnumParser()
{
    enumMap["Value1"] = Value1;
    enumMap["Value2"] = Value2;
}

enum OtherEnum
{
    Value3, 
    Value4
};
EnumParser<OtherEnum>::EnumParser()
{
    enumMap["Value3"] = Value3;
    enumMap["Value4"] = Value4;
}

int main()
{
    EnumParser<SomeEnum> parser;
    cout << parser.ParseSomeEnum("Value2");
}

Answer 2

std::map< string, enumType> enumResolver;

Answer 3

Accepted answer doesn't contain full listing. I'm adding EnumParser.h which I created from the accepted answer, hope it can help

#include <string>
#include <map>

using namespace std;

template <typename T> class EnumParser
{
    map<string, T> enumMap;
public:
    EnumParser(){};

    T ParseSomeEnum(const string &value)
    { 
        typename map <string, T>::const_iterator iValue = enumMap.find(value);
        if (iValue  == enumMap.end())
            throw runtime_error("");
        return iValue->second;
    }
};

Usage is simple:

enum FieldType
{
    Char,
    Integer,
    Long,
    Fixed,
    Price,
    Date,
    Time
};

EnumParser<FieldType>::EnumParser()
{
    enumMap["Char"] = Char;
    enumMap["Integer"] = Integer;
    enumMap["Long"] = Long;
    enumMap["Fixed"] = Fixed;
    enumMap["Price"] = Price;
    enumMap["Date"] = Date;
    enumMap["Time"] = Time;
}

use:

 EnumParser<FieldType> fieldTypeParser;
 FieldType val = fieldTypeParser.ParseSomeEnum(stringValue)

Answer 4

I agree with many of the answers that std::map is the easiest solution.

If you need something faster, you can use a hash map. Perhaps your compiler already offers one, such as hash_map or the upcoming standard unordered_map, or you can get one from boost. When all the strings are known ahead of time, perfect hashing can be used as well.

Answer 5

Using a std::map raises the question: how does the map get initialised? I would rather use a function:

enum E { A, B };

E f( const std::string & s ) {
   if ( s == "A" ) {
      return A;
    }
    else if ( s == "B" ) {
      return B;
    }
    else {
      throw "Your exception here";
    }
}

Answer 6

Look at Boost.Bimap, it provides bidirectional associations between two sets of values. You can also choose the underlying container.

Answer 7

Is this what you want? The initialization is straight, and no instantiation is needed.

usage:

enum SomeEnum
{
    ENUM_ONE,
    ENUM_TWO,
    ENUM_THREE,
    ENUM_NULL
};

DEFINE_PAIRLIST(CEnumMap, SomeEnum)

INIT_PAIRLIST(CEnumMap)=
{
        {"One", ENUM_ONE},
        {"Two", ENUM_TWO},
        {"Three", ENUM_THREE},
        {"", ENUM_NULL}
};

main{
    // Get enum from string
    SomeEnum i = CEnumMap::findValue("One");

    // Get string from enum
    SomeEnum eee = ENUM_ONE;
    const char* pstr = CEnumMap::findKey(eee);
    ...
}

library:

template <class T>
struct CStringPair
{
    const char* _name;
    T _value;
};

template <class T, class Derived>
struct CStringPairHandle
{
    typedef CStringPair<T> CPair;
    static const CStringPair<T> * getPairList(){
        return Derived::implementation();
    }
    static T findValue(const char* name){
        const CStringPair<T> * p = getPairList();
        for (; p->_name[0]!=0; p++)
            if (strcmp(name,p->_name)==0)
                break;
        return p->_value;
    }

    static const char* findKey(T value){
        const CStringPair<T> * p = getPairList();
        for (; p->_name[0]!=0; p++)
            if (strcmp(value,p->_value)==0)
                break;
        return p->_name;
    };
};

#define DEFINE_PAIRLIST(name, type) struct name:public CStringPairHandle<type, name>{ \
    static CPair _pairList[];       \
    static CPair* implementation(){     \
        return _pairList;           \
    }};
#define INIT_PAIRLIST(name) name::CPair name::_pairList[]

Answer 8

You can calculate the hash of the string and then use this:

template <typename H, typename E>
E map_hash(H const key, std::initializer_list<std::pair<H, E>> const il)
{
  auto const i(
    std::find_if(il.begin(),
      il.end(),
      [key](auto& p)
      {
        return p.first == key;
      }
    )
  );

  assert(i != il.end());

  return i->second;
}

Answer 9

Using C++ reflection library from here: https://github.com/tapika/cppreflect

You can - include library like this:

#include "cppreflect/cppreflect.h"

Basic usage:

Declare enumeration:

DECLARE_ENUM( enumName,
    // Prefix for all enums, "" if no prefix used.
    "myenum_",

    myenum_enumValue1,
    myenum_enumValue2,
    myenum_enumValue3 = 5,

    // comment
    myenum_enumValue4
);

Conversion logic:

From enumeration to string:

printf( EnumToString(myenum_enumValue3).c_str() );

=> "enumValue3"

From string to enumeration:

enumName value;

if( !StringToEnum("enumValue4", value) )
    printf("Conversion failed...");

=> 

value == myenum_enumValue4

Main / core functionality resides in here:

https://github.com/tapika/cppreflect/blob/master/cppreflect/enumreflect.h

Answer 10

Parse the string yourself, match the string with a value (which is also an index to a map<string, enum>.