Question

First off I am new to C++, especially using C++ in an OOP fashion. I have a class with multiple subclasses, and I was wondering if I could ambiguously declare a variable to accept an object without limiting the which objects can be stored in it. I am asking because one of the multiple children will end up being used at a time. So if I cannot ambiguously declare a variable I a way to determine which of the numerous variables are in use.

Something along the lines of

obj randomObj = new className;

instead of

className randomObj = new className

Answer 1

You say you’re new to C++, and the syntax you use to describe what you want suggests you’re more familiar with another languages like Java or C#. The syntax you show works fine in those languages:

Foo myFoo = new DerivedFoo;

This works because, behind the scenes in Java and C# myFoo actually behaves as a pointer to a Foo rather than as name for a fixed memory region capable of storing a Foo. In C++ the syntax Foo myFoo creates such a fixed memory region. Even if you try to put some derived type there by doing this:

DerivedFoo myDerivedFoo;
Foo myFoo = myDerivedFoo;

myFoo can still only hold a Foo object. Everything that’s not a Foo is ‘sliced’ away during the initialization of myFoo, because it just doesn’t fit in that fixed memory region..

So in C++ you have to explicitly do what Java and C# do behind the scenes by using C++’s pointer syntax:

    Foo *myFoo = new DerivedFoo;

Now myFoo is a pointer to a Foo, and that pointer can refer to a any Foo object, including the Foo parts of a DerivedFoo, without any slicing or anything occurring. new DerivedFoo creates a memory region where a DerivedFoo can exist, and then myFoo is set to point at the Foo part of the created DerivedFoo.