c++: Using X Macro to define enum and string array inside class?

Question

Using C++ (Visual Studio). I'm trying to find a solution for converting an enum to a string. I came across X Macros (http://drdobbs.com/cpp/184401387) which seems to be a reasonable solution but I'm having a hard time getting it to work inside of a class. All the examples I've seen show everything defined outside of a class.

// ColorNames.h
X(Red, "Red"),
X(Blue, "Blue"),
...


// MyClass.h
class MyClass
{
public:
    MyClass();
    virtual ~MyClass();

    #define X(a, b) a
    enum Colors {
        #include "ColorNames.h"
    };
    #undef X

    #define X(a, b) b
    char *colorNameStrings_[] = {
        #include "ColorNames.h"
    };
    #undef X
}

The IDE chokes on the line *colorNameStrings_[] =... I guess because you can't initialize a data member variable in the header file? How do I get this to work?

My goal is to have an enum to string mapping. The idea of X Macros, I believe, is to help guarantee that mapping stays in sync. — User, Sep 08 '11 at 20:57

Etienne de Martel · Accepted Answer · 2011-09-09T14:09:38.863

The problem is that you can't initialize a non static constant inside a class definition.

You would probably have to do it like that:

// MyClass.h
class MyClass
{
public:
    MyClass();
    virtual ~MyClass();

    #define X(a, b) a
    enum Colors {
        #include "ColorNames.h"
    };
    #undef X

    static const char * colorNameStrings_[];
};

And in the .cpp file:

// MyClass.cpp
#include "MyClass.h"

#define X(a, b) b
const char * MyClass::colorNameStrings_[] = {
    #include "ColorNames.h"
};
#undef X

score 2 · Answer 2 · edited May 23 '17 at 12:06

2

Instead of using X-Macros, consider using the Boost.Preprocessor library. The initial, one-time definition of the code generation macros takes a bit of work, but the end result is far cleaner and much easier to reuse.

I provided a basic implementation of a generic, Boost.Preprocessor-based enum-to-string-name conversion in an answer to "How to convert an enum type variable to a string?"

edited May 23 '17 at 12:06

Community

1
1

answered Sep 08 '11 at 21:00

James McNellis

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Tony The Lion · Answer 3 · 2011-09-08T21:24:04.577

1

Use a struct like this

struct convert
{
     std::map<MyEnum, std::string> mapping;

     convert() 
     { 
          mapping[SOME_ENUM_VALUE] = "SomeValue"; 
          // etc to fill map
     }

     std::string operator()(MyEnum enum)
     {
         return mapping[enum];    
     }

};

Then use like this:

convert c;
std::string ret = c(myenum);  //calls operator()

edited Sep 08 '11 at 21:24

answered Sep 08 '11 at 20:56

Tony The Lion

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That is ridiculously verbose. Also, why use a struct when a function would do? – Etienne de Martel Sep 08 '11 at 20:57
@Etienne, that's just how I used to do it, when I had loads of enums in need of conversion. – Tony The Lion Sep 08 '11 at 21:00
1

A benefit of doing it with a struct is that you can define them in more varied scope. With a function you are restricted from inline in a function and fully in a header. – Tom Kerr Sep 08 '11 at 21:07
The basic problem with this that X Macros tries to solve is that the programmer must remember to keep the enum and the std::map in sync. – User Sep 08 '11 at 22:21

c++: Using X Macro to define enum and string array inside class?

3 Answers3