c++ - Why upcasting works but downcasting gives compile time error? -

c++ - Why upcasting works but downcasting gives compile time error? -

to utilize dynamic_casting, inheritance hierarchy needs polymorphic. in below code segment, dynamic_cast works upcasting while fails @ compile time downcasting? what's reason upcasting works?

class b{}; class c : public b{}; int main() { c* pcob = new c(); b* bp = dynamic_cast<b*>(pcob); // #1 : upcasting works if(bp) cout << "casted" << endl; else cout << "bp null" << endl; delete bp; b* pbob = new b(); c* pc = dynamic_cast<c*>(pbob); // #2 : downcasting gives error if(pc) cout << "casted" << endl; else cout << "pc null" << endl; delete pbob; homecoming 0; }

the compile time error @ #2 is

main.cpp: in function ‘int main()’: main.cpp:36:34: error: cannot dynamic_cast ‘pbob’ (of type ‘class b*’) type ‘class c*’ (source type not polymorphic) c* pc = dynamic_cast<c*>(pbob);

the standard's been quoted - praise be.

to thought why standard specifies behaviour does, helps consider you're asking compiler , how relates mutual implementations of runtime type information....

your dynamic_cast<b*>(pcob) needs know offset of b relative containing c - that's constant amount every c object, if c embedded in other object. there's no reliance on unknown until runtime.

contrast dynamic_cast<c*>(pbob) b* pbob - compiler can't know whether pointed-to b embedded in object @ all, allow lone whether object happens - or derived - c, such cast can succeed. in general, needs rely on runtime type info generated polymorphic types.

in specific code, compiler knows actual types of objects, , little effort have supported downcast, similar code in function gets b* parameter or calls opaque mill method returning b* may dynamic_cast runtime object containing c , other times not - compiler can't employ "localised" knowledge of real types. in local usage, can create pointers actual runtime type, there's no utility gained requiring compilers back upwards "localised" usage, , standard bans such casts wholesale.

c++ casting