How to Efficiently Convert Enum Types to Strings in C and C ?

Converting Enum Types to Strings: A Comprehensive Guide

Converting enum type variables to strings is a common requirement in programming. For instance, it becomes necessary when printing enum values in a readable format or when passing them as parameters to functions that expect string arguments.

The Naive Approach: Manual Conversions

One straightforward approach to enum type conversion is to write a manual function for each enum. For example, to convert an OS_type enum:

enum OS_type { Linux, Apple, Windows };

inline const char* ToString(OS_type v)
{
    switch (v)
    {
        case Linux:   return "Linux";
        case Apple:   return "Apple";
        case Windows: return "Windows";
        default:      return "[Unknown OS_type]";
    }
}

However, this approach is prone to maintenance issues as the number of enums increases.

A Preprocessor-Based Solution with Boost.Preprocessor

Boost.Preprocessor library provides a more elegant and automated way to handle enum type conversions. It utilizes the preprocessor to generate conversion functions dynamically. The following two macros achieve this:

#define X_DEFINE_ENUM_WITH_STRING_CONVERSIONS_TOSTRING_CASE(r, data, elem) \
    case elem : return BOOST_PP_STRINGIZE(elem);

#define DEFINE_ENUM_WITH_STRING_CONVERSIONS(name, enumerators) \
    enum name { \
        BOOST_PP_SEQ_ENUM(enumerators) \
    }; \
    \
    inline const char* ToString(name v) \
    { \
        switch (v) \
        { \
            BOOST_PP_SEQ_FOR_EACH( \
                X_DEFINE_ENUM_WITH_STRING_CONVERSIONS_TOSTRING_CASE, \
                name, \
                enumerators \
            ) \
            default: return "[Unknown " BOOST_PP_STRINGIZE(name) "]"; \
        } \
    }

Using these macros, the OS_type enum can be defined as follows:

DEFINE_ENUM_WITH_STRING_CONVERSIONS(OS_type, (Linux)(Apple)(Windows))

Usage in C and C

In C , the ToString function can be used to convert enum values to strings:

#include <iostream>

int main()
{
    OS_type t = Windows;
    std::cout << ToString(t) << " " << ToString(Apple) << std::endl;
}

For C, the ToString function can be implemented as a macro instead of a function overload:

#define ToString(t) \
    [Unknown ""##t] [Linux "Linux"] [Apple "Apple"] [Windows "Windows"]

Conclusion

This article has presented two methods for converting enum type variables to strings: the naive approach using manual functions and a preprocessor-based solution with Boost.Preprocessor. The preprocessor-based approach is more robust and easier to maintain, especially when dealing with large numbers of enums.

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