Declare object of derived class without specifying it

There is a base-class Base and two possible derived classes DerivedA and DerivedB. How can I declare a variable without specifying (at the point of declaration) which of the two derived classes will be in use?

I have tried the following example:

#include <iostream>
#include <vector>
#include <memory>
using namespace std;
#include <stdlib.h>


struct Base
{
    int base = 0;
};

struct DerivedA : Base
{
    int x = 1;
};

struct DerivedB : Base
{
    int y = 1;
};


class Test
{
public:
    Test(int a)
    {
    Base TestObj;
    if (a==0)
    {
        DerivedA TestObj; // intention: change type of TestObj to DerivedA
    }
    else
    {
        DerivedB TestObj; // intention: change type of TestObj to DerivedB
    }

    TestObj.base = 7;

    if (a==0)
    {
        TestObj.x = 2;
    }
    else
    {
        TestObj.y = 4;
    }

    myObjs.push_back(make_shared<Base>(TestObj));
    }
private:
    vector<shared_ptr<Base>> myObjs;

};

The compiler will throw an error saying "error: ‘struct Base’ has no member named ‘x’ " (or 'y' respectively).

A trivial solution would be to include everything in the first if (a==0) / else statements using a separate myObjs.push_back call for the two cases. However, I am interested in a solution where I can stay more flexible, ideally by e.g. only changing the line 'Base TestObj;' to something more general. Thanks in advance.