On 12/08/10 18:14, Steven Watanabe wrote:

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.

Why is downcasting needed instead of just eliminating the implicit upcasting to const S&. IOW, instead of: const S& s = d3; my_variant_t v = s; why not: my_variant_t v = d3; ? WARNING: not tested. -Larry