java - Instantiating derived types in C++ -

imagine have base class, , 2 classes derive it, 1 , two. in java, have following scenario:

base b; if(condition)   b = new one(); else   b = new two();    

where object type determined @ runtime (the above objects go on heap).

in c++, want able instantiate object type @ runtime - know both share same base type - want keep stack allocated, so:

base b; 

what's best way this?

what's best way this?

you can't. if declare variable type base, stack allocation suitable holding instance of base not instance of derived type (which might larger, though if not, still cannot ask; runtime type of variable in c++ same declared type). @ best, slice derived instance base-type variable.

the best bet use pointer, optionally wrapped in shared_ptr or unique_ptr give similar semantics (i.e. have object automatically destroyed when goes out of scope, assuming ownership hasn't been transferred).

base* b = (condition) ? (base *) new one() : new two(); auto bptr = shared_ptr<base>(b); 

note gives same java. object heap allocated, reference stack allocated. despite syntax, reference-type java variable equivalent pointer in c++.