CODEWITHSUNDEEP
c++stringenums
Daniel
Daniel

Reputation: 6745

C++ string to enum

Is there a simple way in C++ to convert a string to an enum (similar to Enum.Parse in C#)? A switch statement would be very long, so I was wondering if there is a simpler way to do this?

EDIT:

Thanks for all of your replies. I realized that there was a much simpler way to do it for my particular case. The strings always contained the charater 'S' followed by some number so i just did

int i = atoi(myStr.c_str() + 1);

and then did a switch on i.

Upvotes: 38

Views: 156992

Answers (13)

glades
glades

Reputation: 4737

You will have to map the string values (chain of characters) to a corresponding enum value (integer). Here's how you would do this in practice using std::map:

Demo

#include <cstdio>
#include <map>
#include <string_view>
#include <stdexcept>

enum class myenum
{
    content_encoding,
    content_length,
    transfer_encoding,
};

// In C++23 use a constexpr flat_map instead
const std::map<std::string_view, myenum> map_myenum = {
    {"content-encoding", myenum::content_encoding},
    {"content-length", myenum::content_length},
    {"transfer-encoding", myenum::transfer_encoding},
};

auto test(std::string_view str)
{
    try {
        switch(map_myenum.at(str)) {
            case myenum::content_encoding:
                printf("String was content_encoding\n");
                break;
            case myenum::content_length:
                printf("String was content_length\n");
                break;
            case myenum::transfer_encoding:
                printf("String was transfer_encoding\n");
                break;
        }
    } catch(const std::out_of_range& e) {
        printf("String didn't match any criteria!\n");
    }
    
}

int main()
{
    test("content-encoding");
    test("content-length");
    test("some random other stuff");
}

Output:

String was content_encoding
String was content_length
String didn't match any criteria!

A few notes:

  • We use a const map to hint to the compiler that this map isn't supposed to be changed at all. The exception handling is a necessary evil as for const maps we can only use the .at() method as it doesn't insert items if it doesn't find the key in the map. It's a bit of an unfortunate decision by the CWG in my view, as this is definitely nothing "exceptional" and we shouldn't have to pay the cost (in this case at least).
  • In C++23 we get flat_map which is more suitable in this case, as I suppose it will contain constexpr initializers and assignement which would allow us to lay out the whole map at compile time, so that we just pay the overhead of copy-pasting the final map to RAM. It's also a lot more cache-friendly.

Upvotes: 0

Andrii Syrokomskyi
Andrii Syrokomskyi

Reputation: 423

I use this "trick" > http://codeproject.com/Articles/42035/Enum-to-String-and-Vice-Versa-in-C

After

enum FORM {
    F_NONE = 0,
    F_BOX,
    F_CUBE,
    F_SPHERE,
};

insert

Begin_Enum_String( FORM )
{
    Enum_String( F_NONE );
    Enum_String( F_BOX );
    Enum_String( F_CUBE );
    Enum_String( F_SPHERE );
}
End_Enum_String;

It works fine, if the values in the enum are not duplicates.

Example in code

enum FORM f = ...
const std::string& str = EnumString< FORM >::From( f );

vice versa

assert( EnumString< FORM >::To( f, str ) );

Upvotes: 7

Mark Ransom
Mark Ransom

Reputation: 308081

A std::map<std::string, MyEnum> (or unordered_map) could do it easily. Populating the map would be just as tedious as the switch statement though.

Edit: Since C++11, populating is trivial:

static std::unordered_map<std::string,E> const table = { {"a",E::a}, {"b",E::b} };
auto it = table.find(str);
if (it != table.end()) {
  return it->second;
} else { error() }

Upvotes: 56

Alexander I. Denisevich
Alexander I. Denisevich

Reputation: 11

"Additional question: Is it possibile to handle undefined strings ? I mean if I try to get the value for responseHeaderMap["cookie"], what will be the value? (provided that "cookie" is not defined in the responseHeaderMap – bart s Nov 22 '16 at 12:04"

well, you can just make check before:

auto it = responseHeaderMap.find("cookie");
if (it != responseHeaderMap.end())
{
     // "cookie" exist, can take value 
}

After "cookie" exist check, you can get it value with use:

responseHeaderMap["cookie"]

hope this help

Upvotes: 0

Guy L
Guy L

Reputation: 2954

saw this example somewhere

#include <map>
#include <string>

enum responseHeaders
{
    CONTENT_ENCODING,
    CONTENT_LENGTH,
    TRANSFER_ENCODING,
};

// String switch paridgam   
struct responseHeaderMap : public std::map<std::string, responseHeaders>
{
    responseHeaderMap()
    {
        this->operator[]("content-encoding") =  CONTENT_ENCODING;
        this->operator[]("content-length") = CONTENT_LENGTH;
        this->operator[]("transfer-encoding") = TRANSFER_ENCODING;
    };
    ~responseHeaderMap(){}
};

Upvotes: 14

Mara Black
Mara Black

Reputation: 1751

this worked for me: 

enum NODES { Cone = 1, BaseColor = 2, NONE = 0 };

std::map<std::string, NODES> nodeMap;
nodeMap["Cone"] = NODES::Cone;
nodeMap["BaseColor"] = NODES::BaseColor;

Upvotes: 8

cprogrammer
cprogrammer

Reputation: 5665

There is no "built-in way", but there are ways to achieve this by storing the pair value-name in an array

enum myEnum
{
    enumItem0,
    enumItem1,
    enumItem7 = 7,
    enumItem8
};

std::vector<std::pair<myEnum,std::string>>   gMap;

#define ADDITEM(x)  gMap.push_back(std::pair<myEnum,std::string>(x,#x));

.....

ADDITEM(enumItem0);
ADDITEM(enumItem1);
ADDITEM(enumItem7);
ADDITEM(enumItem8);

Upvotes: 6

mwigdahl
mwigdahl

Reputation: 16578

No, you'll have to use an if/then construction, or use a map or hash table or some other type of associative data structure to facilitate this.

Upvotes: 0

jj1
jj1

Reputation: 155

You can use macro to minimize repeating yourself. Here is the trick: Enums, Macros, Unicode and Token-Pasting

Upvotes: 2

TeaWolf
TeaWolf

Reputation: 714

While there is no direct solution, there are a few possible workarounds.

Take a look at this question: Easy way to use variables of enum types as string in C?

Upvotes: 2

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361264

Use std::map<std::string, Enum> and use boost::map_list_of to easily initialize it.

Example,

enum X
{
   A,
   B,
   C
};

std::map<std::string, X> xmap = boost::map_list_of("A", A)("B", B)("C",C);

Upvotes: 27

Alexander Sulfrian
Alexander Sulfrian

Reputation: 3563

It is not possible because the names are not available at runtime. During compilation each enum is replaced with the corresponding integer value.

Upvotes: 2

Constantinius
Constantinius

Reputation: 35039

In short: there is none. In C++ enums are static values and not objects like in C#. I suggest you use a function with some if else statements.

Upvotes: 4

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