C++ Bitflaged enum to string

Question

I'm trying to do what Intellisense does in visual studio when you hover over a bitwise-enum (or however it's called) variable (while debugging), by taking an enum and converting it to string.

for example:

#include <iostream>

enum Color {
    White = 0x0000,
    Red = 0x0001,
    Green = 0x0002,
    Blue = 0x0004,
};

int main()
{
    Color yellow = Color(Green | Blue);
    std::cout << yellow << std::endl;
    return 0;
}

If you hover over yellow you'll see:

So I want to be able to call something like:

std::cout << BitwiseEnumToString(yellow) << std::endl;

and have the output print: Green | Blue.

I wrote the following which tries to provide a generic way of for printing an enum:

#include <string>
#include <functional>
#include <sstream>

const char* ColorToString(Color color)
{
    switch (color)
    {
    case White:
        return "White";
    case Red:
        return "Red";
    case Green:
        return "Green";
    case Blue:
        return "Blue";
    default:
        return "Unknown Color";
    }
}

template <typename T>
std::string BitwiseEnumToString(T flags, const std::function<const char*(T)>& singleFlagToString)
{
    if (flags == 0)
    {
        return singleFlagToString(flags);
    }

    int index = flags;
    int mask = 1;
    bool isFirst = true;
    std::ostringstream oss;
    while (index)
    {
        if (index % 2 != 0)
        {
            if (!isFirst)
            {
                oss << " | ";
            }
            oss << singleFlagToString((T)(flags & mask));
            isFirst = false;
        }

        index = index >> 1;
        mask = mask << 1;
    }
    return oss.str();
}

So now I can call:

int main()
{
    Color yellow = Color(Green | Blue);
    std::cout << BitwiseEnumToString<Color>(yellow, ColorToString) << std::endl;
    return 0;
}

I get the desired output.

I'm guessing that I couldn't find anything about it since I don't know how it's called, but anyways -

1. Is there something in std or boost that does that or can be used to provide this?

2. If not, what's the most efficient way to do such a thing? (or would mine suffic)

Answer 1

EDIT: See below for a generic, template implementation... Note, though, that this template implementation tramples ALL OVER ostream's operator <<() implemantations for practically everything! It'd be better if the enums were full-blown classes, with a base class implementation for the template. This generic definition is the equivalent of an atom bomb in a china shop...

---

I wrote the following example, with a test function. It uses C++ overloads to allow you to simply cout a Color - if you want to be able to still print the simple numerical value, you would have to cast it into an int:

#include <iostream>

enum Color {
    White = 0x0000,
    Red   = 0x0001,
    Green = 0x0002,
    Blue  = 0x0004,
}; // Color

std::ostream &operator <<(std::ostream &os, Color color) {
    static const char *colors[] = { "Red", "Green", "Blue", 0 }; // Synchronise with Color enum!

    // For each possible color string...
    for (const char * const *ptr = colors;
         *ptr != 0;
         ++ptr) {

        // Get whether to print something
        bool output = (color & 0x01)!=0;

        // Is color bit set?
        if (output) {
            // Yes! Output that string.
            os << *ptr;
        } // if

        // Next bit in color
        color = (Color)(color >> 1);

        // All done?
        if (color == 0) {
            // Yes! Leave
            break;
        } // if

        // No, so show some more...
        if (output) {
           // If output something, need 'OR'
           os << " | ";
        } // if
    } // for
    return os;
} // operator <<(Color)

void PrintColor() {
    for (unsigned c = 0; c < 8; ++c) {
        Color color = Color(c);
        std::cout << color << std::endl;
    } // fors
} // PrintColor()

---

Generic implementation, with examples

First, a header file:

// EnumBitString.h

template <typename ENUM>
const char * const *Strings() {
    static const char *strings[] = { "Zero", 0 }; // By default there are no Strings
    return strings;
} // Strings<ENUM>()

template <typename ENUM>
std::ostream &operator <<(std::ostream &os, ENUM e) {
    const char * const *ptr = Strings<ENUM>();
    if (e == 0) {
        os.operator <<(*ptr);
        return os;
    } // if

    // For each possible ENUM string...
    while (*ptr != 0) {
        bool output = (e & 0x01) != 0;

        // Is bit set?
        if (output) {
            // Yes! Output that string.
            os.operator <<(*ptr);
        } // if

        // Next bit in e
        e = (ENUM)(e >> 1);

        // All done?
        if (e == 0) {
            // Yes! Leave
            break;
        } // if

        // No, so show some more...
        if (output) {
            os.operator <<(" | ");
        } // if

        ++ptr;
    } // while
    return os;
} // operator <<(ENUM)

Next, your example:

// Colors.h

#include "EnumBitString.h"

enum Colors {
    White = 0x0000,
    Red   = 0x0001,
    Green = 0x0002,
    Blue  = 0x0004,
    NumColors = 4
}; // Colors

template <>
const char * const *Strings<Colors>() {
    static const char *strings[] { "White", // Zero case
                                   "Red",
                                   "Green",
                                   "Blue",
                                   0 }; // Don't forget final 0
    static_assert((sizeof(strings)/sizeof(strings[0])==NumColors+1, "Colors mismatch!");
    return strings;
} // Strings<Colors>()

Then, another example of bits within a value:

// Flags.h

#include "EnumBitString.h"

enum Flags {
    CF = 0x0001,
//  Res1 = 0x02,
    PF = 0x0004,
//  Res2 = 0x08,
    AF = 0x0010,
//  Res3 = 0x20,
    ZF = 0x0040,
    NumFlags = 7
}; // Flags

template <>
const char * const *Strings<Flags>() {
    static const char *strings[] =  { "None",
                                      "Carry",
                                      "",
                                      "Parity",
                                      "",
                                      "Arithmetic",
                                      "",
                                      "Zero",
                                      0 }; // Don't forget final 0
    static_assert((sizeof(strings)/sizeof(strings[0])==NumFlags+1, "Flags mismatch!");
    return strings;
} // Strings<Flags>()

Finally, a test program:

#include <iostream>

#include "Colors.h"
#include "Flags.h"

void TestENUM() {
    for (unsigned c = 0; c < 0x0008; ++c) {
        Colors color = Colors(c);
        std::cout << color << std::endl;
    } // for
    for (unsigned f = 0; f < 0x0080; ++f) {
        Flags flag = Flags(f);
        std::cout << flag << std::endl;
    } // for
} // TestENUM()

Cool, huh?

Answer 2

You're going to have to maintain a list of the string representations of your enum, be it in a vector, hard-coded, etc. This is one possible implementation.

enum Color : char
{
  White = 0x00,
  Red   = 0x01,
  Green = 0x02,
  Blue  = 0x04,
  //any others
}

std::string EnumToStr(Color color)
{
  std::string response;

  if(color & Color::White)
    response += "White | ";
  if(color & Color::Red)
    response += "Red | ";
  if(color & Color::Green)
    response += "Green | ";
  if(color & Color::Blue)
    response += "Blue | ";
  //do this for as many colors as you wish

  if(response.empty())
    response = "Unknown Color";
  else
    response.erase(response.end() - 3, response.end());

  return response;
}

Then make another EnumToStr function for each enum you want to do this to, following the same form