why receive following error following code?
1>c:\libs\boost_1_44\boost/smart_ptr/shared_ptr.hpp(259): error c2683: 'dynamic_cast' : 'my_namespace::a' not polymorphic type 1> d:\[location]\[header_filename].h(35) : see declaration of 'my_namespace::a' 1> c:\libs\boost_1_44\boost/smart_ptr/shared_ptr.hpp(522) : see reference function template instantiation 'boost::shared_ptr<t>::shared_ptr<my_namespace::a>(const boost::shared_ptr<my_namespace::a> &,boost::detail::dynamic_cast_tag)' being compiled 1> 1> [ 1> t=my_namespace::b 1> ] 1> [location]\[source_filename].cpp(217) : see reference function template instantiation 'boost::shared_ptr<t> boost::dynamic_pointer_cast<my_namespace::b,striker::a>(const boost::shared_ptr<my_namespace::a> &)' being compiled 1> 1> [ 1> t=my_namespace::b 1> ] 1>c:\libs\boost_1_44\boost/smart_ptr/shared_ptr.hpp(260): fatal error c1903: unable recover previous error(s); stopping compilation the c++ code more or less following:
#include <list> #include "boost/pointer_cast.hpp" #include "boost/shared_ptr.hpp" struct { public: a(const myenum an_enum_, const int an_int_) : an_enum(an_enum_), an_int(an_int_) {} const myenum an_enum; const int an_int; }; struct b : public { public: b(const int some_int_, const mystruct &a_struct_) : a(enum_option_a, an_int_), a_struct(a_struct_) {} const mystruct a_struct; }; // ussage in function: // ... boost::shared_ptr<a> a_ptr = boost::shared_ptr<a>( new b() ); std::list<boost::shared_ptr<a>> a_list; a_list.push_back(a_ptr); // ... boost::shared_ptr<a> a_ptr2 = a_list.front(); boost::shared_ptr<b> b_ptr = boost::dynamic_pointer_cast<b>(a_ptr2); // <-- error here // ...
dynamic_cast works polymorphic class. , polymorphic class has atleast 1 virtual function, destructor.
//polymorphic classes struct { virtual ~a(); //even virtual destructor makes class polymorphic! }; struct b : { void f(); }; //non-polymorphic classes struct c { ~c(); //not virtual }; struct d : c { void f(); //not virtual either }; in above code, a , b polymorphic classes, c , d not.
a *pa = new b(); b *pb = dynamic_cast<b*>(pa); //okay c *pc = new d(); d *pd = dynamic_cast<d*>(pc); //error - not polymorphic class note in dynamic_cast, source type need polymorphic in order compile. if destination isn't polymorphic, dynamic_cast return null pointer.
d *pd = dynamic_cast<d*>(pa); //okay - source (pa) polymorphic if ( pd ) cout << "pd not null" ; else cout << "pd null"; output:
pd null online demo : http://www.ideone.com/yesxc
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