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

On 2010-09-01 15:30, Stefan Strasser wrote:

Zitat von Roland Bock :

...

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, 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 : mpl::true_{};

cerr << is_boost_optional::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 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 is_convertible // with T being a Field is_convertible // with T being a Field and Field::value_type being an optional is_convertible // 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