Create an enum of std::vector

Question

I am trying to use enumerations of std::vector<bool> because i'd like to represent some values as a vector of bits.

So I tried the following code:

enum class ProximityStateEnum : std::vector<bool> {

    Unknown = std::vector<bool>{false,false},

    NotConnected = std::vector<bool>{false,true},

    Connected = std::vector<bool>{true,false},

    ConnectedButNotLatched = std::vector<bool>{true,true}

};

But when I compile the code with this, I get the error underlying type ‘std::vector<bool>’ of ‘ProximityStateEnum’ must be an integral type. How can I do to create an enum of vectors ?

Vittorio Romeo · Accepted Answer · 2017-11-18 12:28:44Z

4

enum and enum class are not meant as aliases to instances of arbitrary objects - they're limited to integral types, and every explicit value must be a constant expression. See the cppreference page on enum for more information.

If you want to give a name to particular instances of std::vector<bool>, either use functions or inline variables:

auto Unknown()                { return std::vector<bool>{false,false}; }
auto NotConnected()           { return std::vector<bool>{false,true}; }
auto Connected()              { return std::vector<bool>{true,false}; }
auto ConnectedButNotLatched() { return std::vector<bool>{true,true}; }

// ...or, in C++17:
inline auto Unknown =                std::vector<bool>{false,false};
inline auto NotConnected =           std::vector<bool>{false,true};
inline auto Connected =              std::vector<bool>{true,false};
inline auto ConnectedButNotLatched = std::vector<bool>{true,true};

In your particular use case, you don't really need an std::vector because you know the size of your bitset in advance - std::bitset or binary literals will work well:

enum class ProximityStateEnum : /* some suitable type */ 
{
    Unknown =                0b00,
    NotConnected =           0b01,
    Connected =              0b10,
    ConnectedButNotLatched = 0b11
};

answered Nov 18, 2017 at 12:28

Vittorio Romeo

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1 Comment

Xatyrian Over a year ago

Thanks for the answer. Indeed, std::bitset could also be usable but I previously decided to stick with vectors so now I can't really change. Your approach without the enum is interesting, but as I need to group my different values, I think I'd make them static members of structs

Matteo Italia · Accepted Answer · 2017-11-18 12:33:06Z

4

You cannot; enums can only be based on integral types.

If it can be helpful, since C++14 you can use binary literals to specify their values:

enum class ProximityStateEnum {
    Unknown = 0b00,
    NotConnected = 0b01,
    Connected = 0b10,
    ConnectedButNotLatched = 0b11
};

which is pretty much as clear as your idea, but way more compact and orders of magnitude more efficient than any hypothetical std::vector based solution.

Another possibility that is often used in enums where particular bits have particular meaning is to build them with expressions:

enum ProximityStateEnum {
    Unknown = 0b00,
    NotConnected = 0b01,
    Connected = 0b10,
    ConnectedButNotLatched = Connected | NotConnected
};

edited Nov 18, 2017 at 12:33

answered Nov 18, 2017 at 12:26

Matteo Italia

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3 Comments

Xatyrian Over a year ago

Thanks for this answer. In my case, I really need the type to be of type std::vector<bool>. i will probably take a look at struct with static members.

Matteo Italia Over a year ago

@Xatyrian: sounds like an XY problem, you should probably describe why exactly you need it to be an std::vector<bool>.

Matteo Italia Over a year ago

Also, a struct with only static members doesn't make much sense, use a namespace.

max66 · Accepted Answer · 2017-11-18 12:50:59Z

As said by other, enums can only be integral types.

If the lenght of your vectors of bool is constant (as in you example, where is 2), you can substitute the std::vector<bool> with a std::array<bool, length>. One advantage in using std::array instead of std::vector is that you can define it as constexpr.

So a reasonable (IMHO) alternative is define a enum type based on std::size_t, with sequential values starting from 0,

enum ProximityStateEnum : std::size_t
 {
    Unknown = 0U,
    NotConnected,
    Connected,
    ConnectedButNotLatched
 };

and a static constexpr std::array of std::arrays of bool

// .......................... last value of the enum vvvvvvvvvvvvvvvvvvvvvv
static constexpr std::array<std::array<bool, 2U>, 1U+ConnectedButNotLatched>
   pseArr { { { {false, false} }, { {false, true} },
              { {true, false} },  { {true, true} } } };

that you can use as follows

pseArr[NotConnected][0]

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