Re: [Boost-users] [type_traits]: is_boost_optional?

Subject: Re: [Boost-users] [type_traits]: is_boost_optional?
From: Roland Bock (rbock_at_[hidden])
Date: 2010-09-01 10:26:46

On 2010-09-01 15:30, Stefan Strasser wrote:
> Zitat von Roland Bock <rbock_at_[hidden]>:
>
>> Hi,
>>
>> is there something in Boost.TypeTraits (or somewhere else) that allows
>> me to check if a type is a boost::optional?
>>
>> I'd like to do something like this:
>>
>> template<typename T>
>> typename enable_if<is_boost_optional<T> >, T::value_type>::type
>> foo(const T&);
>
> there is nothing in type traits, but you can easily check for a specific
> (template) type:
>
> template<typename T>
> struct is_optional : mpl::false_{};
>
> template<typename T>
> struct is_optional<optional<T> > : mpl::true_{};

cerr << is_boost_optional<boost::optional<int> >::value << endl;

yields: 0
expected: 1

> I don't see why you can't use an overload for boost::optional in the
> first place though:
>
> typename optional<T>::value_type foo(optional<T> const &);

Hmm. The situation is a bit more complex:

I have operands of the following type

template<typename A, typename B, typename Value> struct Field
{
    typedef Value value_type;

    template<typename T>
    some_sfinae_magic operator==(const T&) const;
}

Value can be any numeric type, a string or an optional numeric/string.
T can be numeric type, string or a Field.

some_magic is supposed to check, if T is compatible to Field, either

is_convertible<Field::value_type, T>

is_convertible<Field::value_type, T::value_type>  // with T being a Field

is_convertible<Field::value_type::value_type, T::value_type>  // with T 
being a Field and Field::value_type being an optional

is_convertible<Field::value_type::value_type, T::value_type::value_type> 
// with T being a Field, both Fields having optional value_type

Since value_type is pretty common, a Field with Value=optional<char> and 
string would be found compatible because string::value_type=char.

That's why I want to check if the Value/T::value_type is an optional.

Thanks for asking, I guess I just found a way to clean it up a little 
while writing the stuff down.

Regards,

Roland