Re: [Boost-users] variant over base and derived classes

Subject: Re: [Boost-users] variant over base and derived classes
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2010-12-08 19:14:36

AMDG

On 12/8/2010 4:04 PM, Hicham Mouline wrote:
> How to determine the actual type of the object stored in a variant?
>
> struct S {
>    virtual ~S()
>    {}
> };
>
> struct Deriv1 : public S {
>    virtual ~Deriv1()
>    {}
> };
>
> struct Deriv2 : public S {
>    virtual ~Deriv2()
>    {}
> };
>
> struct Deriv3 : public Deriv2 {
>    virtual ~Deriv3()
>    {}
> };
>
> typedef boost::variant<S,Deriv1, Deriv2, Deriv3>  my_variant_t;
>
> Deriv3 d3;
>
> struct print_type: public boost::static_visitor<>  {
>    template<typename T>
>    void operator()(const T&) const
>    {
>      std::cout<<  typeid(T).name()<<std::endl;
>    }
> };
>
> const S&  s = d3;
> my_variant_t v = s;
> boost::apply_visitor( print_type(), v );
>
> This prints S.
>
> Is it a truncated copy of d3 that is stored v?

Yes.

> How do I construct v polymorphically so that the real object is stored?

You have to downcast before creating the variant.
boost::variant only uses the static type of the object.

In Christ,
Steven Watanabe