static const std::array in class

Question

I have a templated struct with some enumerations in it and I'd like to make a std::array with the enumerations in it for convenience. Is there any way of doing the following?

template< typename A >
struct someClass{
    enum class State{
        sA,
        sB,
        sC
    }

    static const std::array<State,4> the_states = {{
        State::sA,
        State::sB,
        State::sC
    }};

};

In C++03? or C++11? enum class and std::array<> are available with C++11 only. So I suppose your question needs proper tags. — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Oct 15, 2011 at 18:49
Why are you encapsulating an enum class and then further "abstracting" it through an explicit array? — rubenvb
– rubenvb, Commented Oct 15, 2011 at 18:49
The idea is just for convenience of iterating over the states. — shuttle87
– shuttle87, Commented Oct 15, 2011 at 18:50
shuttle87: then make the enum's underlying type explicit (you're already using strongly types enums anyways) and use regular arithmetic to "iterate". — rubenvb
– rubenvb, Commented Oct 15, 2011 at 18:53

David · Accepted Answer · 2011-10-15 19:06:05Z

6

No. Only static const integral data members can be initialized within a class.

However, you could do this...

template< typename A >
struct someClass
{
    enum State
    {
        sA,
        sB,
        sC
    };

    static const std::array<const State,4> the_states;
};

template<typename A>
const std::array<const someClass::State,4> someClass<A>::the_states = 
{
    someClass::State::sA, 
    someClass::State::sB, 
    someClass::State::sC 
};

edited Oct 15, 2011 at 19:06

answered Oct 15, 2011 at 18:59

David

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1 Comment

rubenvb Over a year ago

C++11 (which obviously is in effect here) allows this.

bames53 · Accepted Answer · 2011-10-15 19:16:36Z

#include <iostream>
#include <array>

template< typename A >
struct someClass{
    enum class State {
        sA,
        sB,
        sC
    };

    static const std::array<State,3> the_states;    
};

template<typename A>
const std::array<typename someClass<A>::State,3> someClass<A>::the_states = {
    someClass<A>::State::sA,
    someClass<A>::State::sB,
    someClass<A>::State::sC
};

int main() {
    for( auto i : someClass<int>::the_states) {
        switch(i) {
            case someClass<int>::State::sA:
                std::cout << "sA" << std::endl;
                break;
            case someClass<int>::State::sB:
                std::cout << "sB" << std::endl;
                break;
            case someClass<int>::State::sC:
                std::cout << "sC" << std::endl;
                break;
        }
    }
}

note that you can't terminate the list with 0 the way you were trying with the 4 element array, because 0 cannot be converted to an enum class State.

Bah, and in the time it took me to edit my answer with the real answer Dave got it.

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