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 09:39:50

On 2010-09-01 14:13, Roman Perepelitsa wrote:
> 2010/9/1 Roland Bock <rbock_at_[hidden] <mailto: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&);
>
>
> You can use is_instance_of from lambda library (see
> boost/lambda/detail/is_instance_of.hpp). It's not part of public
> interface though, so use it on your own risk.
>
>    template <class T>
>    struct is_boost_optional : boost::lambda::is_instance_of_1<T,
> boost::optional> {}
>
> Roman Perepelitsa.
>

This seems to work, too:

// -------------------------------------------------------------
template <typename T, typename Enable = void>
class is_boost_optional
{
public:
    static const bool value = false;
};

template <typename T>
class is_boost_optional
    <
       T,
       typename boost::enable_if
       <
          boost::is_same
          <
             boost::optional<typename T::value_type>,
             T
          >
       >::type
    >
{
public:
    static const bool value = true;
};
// -------------------------------------------------------------

Regards,

Roland