Dynamically create on object of type C++

Question

I have a method which returns pointer of one of three relative classes:

Base* Base::search_by_name(string name) { children_iterator = children.begin(); while (children_iterator != children.end()) { if (name == (*children_iterator)->getName()) { switch ((*children_iterator)->class_number) { case 1: return ((Derived*)*children_iterator); case 2: return ((Derived2*)*children_iterator); case 3: return ((Derived3*)*children_iterator); }

And I need to create an object exactly of the class, which class` pointer method returns

 bool Base::set_connection(int number, string& process_object) { typeid(root->search_by_name(process_object)) myobject; // Derived myobject, Derived2 myobject or Derived3 myobject if (myobject != NULL) { string signal = this->getName(); myobject->get_connection (number, signal); //each class has its own realisation of get_connection

I tryed the line typeid(root->search_by_name(process_object)) myobject;But it`s obviously silly. Could you advise something?

You can try using a void pointer and later typecast it accordingly. Not a clean solution though. — DeGo
– DeGo, Commented May 8, 2021 at 21:22

463035818_is_not_an_ai · Accepted Answer · 2021-05-08 21:47:42Z

It's not possible to derive the static type of the polymorphic object so as to make a static declaration of type as in:

my_static_type object;

There are a couple of things you could do.

One is to put a clone() or create() style function in your polymorphic classes to return correctly typed dynamic objects.

BaseClass* n = root->search_by_name(process_object)->create(); // new object

The other would be to manually query the possible types, which rather defeats the point of polymorphism:

auto* p = root->search_by_name(process_object); BaseClass* b; if(auto* n = dynamic_cast<Derived*>(p)) b = new Derived; else if(auto* n = dynamic_cast<Derived2*>(p)) b = new Derived2; else throw std::runtime_error("problems problems");

NOTE: Pointers used for exposition only, use std::unique_ptr etc.. in real code.

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