Ada subtype equivalent in C++

Question

Does C++ offer something similar to Ada\'s subtype to narrow a type?

E.g.:

type Weekday is (Monday, Tuesday, Wednesday, Thursday, Friday,          


        

Answer 1

What you want might (at least partially) be realized using std::variant introduced with C++17.

struct Monday {};
struct Tuesday {};
/* ... etc. */
using WeekDay= std::variant;

The following code defines sub_variant_t which constructs a new variant from the submitted type. E.g. using Working_Day= sub_variant_t; takes the first five elements from Weekday.

template
struct sub_variant_h;

template
struct sub_variant_h >
{
    using type= std::variant... >;
};

template
struct sub_variant
{
    using type= typename sub_variant_h >:type;
};

template
using sub_variant_t = typename sub_variant::type;

If you want to copy values from the smaller type (Working_Day) to the larger one (Weekday) you can use WeekDay d3= var2var( d1 ); where var2var is defined as follows.

template
toT
var2var( std::variant const & v )
{
    return std::visit([](auto&& arg) -> toT {return toT(arg);}, v);
}

See this livedemo.