How to check if enum value is valid?

Question

I am reading an enum value from a binary file and would like to check if the value is really part of the enum values. How can I do it?

Answer 1

Yet another way to do it:

#include 
#include 
#include 

template
struct enum_traits { static constexpr void* values = nullptr; };

namespace detail
{

template
constexpr bool is_value_of(int, void*) { return false; }

template
constexpr bool is_value_of(int v, U)
{
    using std::begin; using std::end;

    return std::find_if(begin(enum_traits::values), end(enum_traits::values),
        [=](auto value){ return value == static_cast(v); }
    ) != end(enum_traits::values);
}

}

template
constexpr bool is_value_of(int v)
{ return detail::is_value_of(v, decltype(enum_traits::values) { }); }

////////////////////
enum Abc { A = 4, B = 8, C = 12 };

template<>
struct enum_traits { static constexpr auto values = { A, B, C }; };
decltype(enum_traits::values) enum_traits::values;

enum class Def { D = 1, E = 3, F = 5 };

int main()
{
    std::cout << "Abc:";
    for(int i = 0; i < 10; ++i)
        if(is_value_of(i)) std::cout << " " << i;
    std::cout << std::endl;

    std::cout << "Def:";
    for(int i = 0; i < 10; ++i)
        if(is_value_of(i)) std::cout << " " << i;
    std::cout << std::endl;

    return 0;
}

The "ugly" part of this approach IMHO is having to define:

decltype(enum_traits::values) enum_traits::values

If you are not opposed to macros, you can wrap it inside a macro:

#define REGISTER_ENUM_VALUES(name, ...) \
template<> struct enum_traits { static constexpr auto values = { __VA_ARGS__ }; }; \
decltype(enum_traits::values) enum_traits::values;