C++ Class that has a custom enum [duplicate]

Question

First english is not my first language. If you need something explained more just ask.

What I want to do, is make a class that extends the use of an enum. Say you have

enum Color 
{ red,
  green, 
  blue };

I want to make a class, so I can make an object like

CustomEnum myEnum("red", 
                  "green", 
                  "blue", 
                  "The house is red", 
                  "The tree is green", 
                  "The car is blue");

This way, I could call something like

myEnum.getString(red);

With a custom function, it would return the mapped value of "The house is red". However I have many many more functions I want to write, and have many enum's.

CustomEnum mySecondEnum("pizza", 
                  "soup", 
                  "eggs", 
                  "burger", 
                  "The pizza is hot", 
                  "The soup has gone cold", 
                  "The eggs are bad",
                  "The burger has cheese");

Notice the two enum's have different sizes, but CustomEnum does not care.

I have done lots of googling and came up with either this cannot be done (because enums are made at compile), or I am googling the wrong things. at this point I want to confirm that this cannot be done or if someone can point me in the right direction.

Other dupe: How to convert an enum type variable to a string?... The premises of the Q has a problem. Apparently, you want to have a single CustomEnum and combine all the enum types like Color, Food etc into that. But how are you going to check if the red and pizza are not same. Hence for every enum type, you may require a separate std::map. For that you may refer the linked & above dupes. — iammilind
– iammilind, Commented Jun 10, 2016 at 6:55

3 revs · Accepted Answer · 2016-06-10 07:00:38Z

1

Given your...

enum Colour { red, green, blue };

You can create a separate mapping:

std::map<Colour, std::string> colour_strings = {
    { red, "red" }, { green, "green" }, { blue, "blue" }
    // or if you like...
    { red, "The house is red" }, { green, "The tree is green" },
    { blue, "The car is blue" }
};

You can then lookup the strings in your code:

Colour my_colour = green;
std::cout << colour_strings[my_colour] << '\n';

There are a variety of "tricks" C++ developers use to make this easier, including:

#define X(NAME) { NAME, #NAME }

std::map<Colour, std::string> colour_strings = {
    X(red), X(green), X(blue)
};

...and...

colours.h

X(red)
X(green)
X(blue)

the_app.cc

// include once to create enum itself...
#define X(IDN) IDN,
enum Colours {
#include "colours.h"
    something_for_after_last_comma
};

// include again to create colour string mappings...
#define X(IDN) { IDN, #IDN }
std::map<Colour, std::string> colour_strings = {
#include "colours.h"
};

edited Jun 10, 2016 at 7:00

community wiki

3 revs
Tony D

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3 Comments

iammilind Over a year ago

To be pedantic, I would either make the color_strings as a const or will not use operator[]. Because OP may not expect to create an empty string, when certain enum value is not mapped [un]knowingly. Also Color as a key for the std::map is not right, because OP expects pizza etc.. In case if pizza and red collide then that will invalidate the whole structure. So the question itself is with less information.

Tony Delroy Over a year ago

@iammilind: making colour_strings const would prevent operator[] being used, so if the program can not be trusted to have valid enumerations then moving away from direct use of map to e.g. const char* str(Colour) is probably better anyway, and I tend to implement an operator<< for the enum type which permits lexical casting too... rapidly gets complex but I think what I've presented is reasonable for a learner asking this question. Re "Color as a key is not right" - the question shows "red" quoted in the CustomEnum but myEnum.getString(red); as client usage - so who knows?

Tony Delroy Over a year ago

Enumerations can certainly have the same value preventing their use as keys, and it's valid for an enum to take on values not matching any enumeration identifier (e.g. commonly a bitwise OR of values) - not saying this works for all enumerations.

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