Adding values to a C++ enum

Question

Suppose I have the following code in another library that I can't change:

typedef enum {
    p0  = 0,
    p1  = 1,
    p2  = 2,
    p3  = 3,
    p4  = 4,
    p5  = 5,
    p6  = 6,
    ...
} PinName;

I want to add some extra aliases like this (not using const PinName PIN_...):

enum class : PinName {
    PIN_SD_MOSI = p0,
    PIN_SD_MISO = p4,
    PIN_SD_SCK = p2,
    PIN_SD_CSN = p6,
};

But it doesn't work. I get the following error:

error: underlying type 'PinName' of '<anonymous enum class>' must be an integral type
  enum class : PinName {
               ^

I also tried using enum class : int { but then the aliases are never in scope - I suspect I have to use plain enum instead. enum : int compiles, but then you can't pass any of the aliases to functions that take PinName. You get this error:

error: no matching function for call to 'foo(<anonymous enum>, <anonymous enum>)'
 foo(PIN_SD_MISO, PIN_SD_MOSI);
     ^

(Candidate is foo(PinName, PinName).)

Does anyone have any idea of a nice solution before I give up and use const PinName PIN_SD_MISO = p2;?

inheritance is not possible in enum classes. See also: [http://stackoverflow.com/questions/644629/base-enum-class-inheritance](http://stackoverflow.com/questions/644629/base-enum-class-inheritance) — wenndemann, Dec 17 '15 at 14:03
maybe [this solution](http://stackoverflow.com/a/14508431/33499). — wimh, Dec 17 '15 at 14:03
"`const PinName PIN_SD_MISO = p2;`" this is the right thing to do. — n. m. could be an AI, Dec 17 '15 at 14:06
Would you consider providing a conversion function as a solution? It somehow defeats the purpose of a strongly typed `enum`. — 5gon12eder, Dec 17 '15 at 14:13
@n.m. As I understand it the const thing would use memory, whereas an enum wouldn't. — Timmmm, Dec 17 '15 at 15:09
@Wimmel Interesting, thanks! Probably not worth the complexity for me but nice link nonetheless. — Timmmm, Dec 17 '15 at 15:10
"would use memory". Not necessarily. Make it static and check. — n. m. could be an AI, Dec 17 '15 at 15:16
Yeah I guess it probably wouldn't - the optimiser should be fairly easily able to work out that I never takes its address or write to it. I guess it was just the principle of the thing! (Btw `static` would be unnecessary since all `const`s are implicitly `static`). — Timmmm, Dec 17 '15 at 15:56
Have you tried using the scope resolution operator: `PIN_SD_MOSI = PinName::p0`? — Thomas Matthews, Dec 17 '15 at 16:43

score 0 · Answer 1 · answered Feb 10 '19 at 00:20

0

Kind of hacky but this is what I came up with if dealing with scoped enums:

enum class OriginalType {
   FOO,  // 0
   BAR   // 1
   END   // 2
};

enum class ExtendOriginalType : std::underlying_type_t<OriginalType> {
   EXTENDED_FOO = static_cast<std::underlying_type_t<OriginalType>>
                                           (OriginalType::END), // 2
   EXTENDED_BAR  // 3
};

and then use like:

OriginalType myOriginalType = (OriginalType)ExtendOriginalType::EXTENDED_BAR;

from Base enum class inheritance

answered Feb 10 '19 at 00:20

jsadler

109
6

`OriginalType` is not an `enum class` – Timmmm Feb 10 '19 at 11:21
I don't understand what you mean. Elaborate? – jsadler Feb 11 '19 at 15:00
In my question `PinName` is an `enum`, not an `enum class`, and I cannot change it. – Timmmm Feb 11 '19 at 15:14

score 0 · Answer 2 · answered Feb 10 '19 at 01:14

0

I will be shot down for this, but why not just use macros?

#define PIN_SD_MOSI  p0
#define PIN_SD_MISO  p4
#define PIN_SD_SCK   p2
#define PIN_SD_CSN   p6

answered Feb 10 '19 at 01:14

TonyK

16,761
4
37
72

I did upvote you but actually, re-reading my question I did already mention this possibility (but using `const PinName PIN_SDMOSI = p2;` which is a better solution). – Timmmm Feb 11 '19 at 15:20
@Timmmm: why is it a better solution? Macros are simpler, after all. – TonyK Feb 11 '19 at 22:54
2

It is properly typed and namespaced. – Timmmm Feb 12 '19 at 12:29
How about `inline constexpr PinName PIN_SD_MISO(){ return p2; }` ? – fana Apr 14 '23 at 08:00

score 0 · Answer 3 · answered Aug 12 '23 at 09:19

I don't see any other "nice" solution besides a set of constants:

constexpr auto PIN_SD_MOSI = p0;
constexpr auto PIN_SD_MISO = p4,
constexpr auto PIN_SD_SCK = p2;
constexpr auto PIN_SD_CSN = p6;

You could also use another unscoped enum:

enum PinName2 {
    PIN_SD_MOSI = p0,
    PIN_SD_MISO = p4,
    PIN_SD_SCK = p2,
    PIN_SD_CSN = p6,
};

The "problem" with scoped enums is that they're their own type, and they don't implicit convert to the underlying type.

Adding values to a C++ enum

3 Answers3