C++ Determine the type of a polymorphic object at runtime

I am trying to work on a project that will require me to determine a polymorphic object's type at runtime, so that I can cast it. An example of what I mean:

class A{
};
class B: public A{
    public:
        void foo(){
            printf("B::foo()\n");
        }
};

Later, I will have a bunch of B objects that are essentially stored as such:

std::vector<A*> v;
v.push_back(new B());

And I will need to call certain overloaded methods defined as:

void bar(B* b){
    b->foo();
}

After passing in objects that are stored in v. The problem that I am having is that in my actual use-case, I don't know the type of B at compile-time, so I can't just call bar by saying bar((B*)v.get(0));

The solution I have been thinking I might need is to somehow determine the type that each object is at runtime, so that I can cast it before passing it to bar.

The solution I have tried so far was to use decltype, but it didn't work for me because it just returns the static type of the value passed in, not the type at runtime.

Also, I do not want to use third party libraries for this project, since I would like to make it as small as possible.

Thank you for your help.