unique pointer of base class with derived member - c++

Question

For my C++ program I have a lot of classes where a member should be of one of two types which have the same base class.
I thought I could implement this with pointers but I don't get it to work.
Example: lets assume we have a class A with a member b_ of class B:
class A{
public:
A(B b): b_{b} {}
private:
B b_;
}
The class B has only one function which is pure virtual:
class B{
public:
virtual void print() = 0;
}
now I have two derived classes of B and I want to change A in a way, that it could hold eihther objects of class B1 or B2:
class B1: public B{
public:
void print(){cout << "B1\n";}
}
class B2: public B{
public:
void print(){cout << "B2\n";}
}
My plan was to use unique pointers:
class A{
public:
A(std::unique_ptr<B> b): b_{std::move(b)} {}
private:
std::unique_ptr<B> b_;
}
int main(){
std::unique_ptr<B> b;
if (some condition){
b = make_unique<B1>(new B1()) ///?
}else{
b = make_unique<B2>(new B2()) ///?
}
A(b);
A.getB()->print();
}

Answer 1

There are several mistakes on your code. First of, you can't have two definitions of A. Second, you must pass the unique_ptr as r-value reference (std::move) since it is not copyable. Last, make a variable of type A (a) and then call methods on it.
#include <memory>
#include <iostream>
using namespace std;
class B{
public:
virtual void print() = 0;
virtual ~B() {};
};
class B1: public B{
public:
void print(){cout << "B1\n";}
};
class B2: public B{
public:
void print(){cout << "B2\n";}
};
class A{
public:
A(std::unique_ptr<B> b): b_{std::move(b)} {}
auto *getB() { return b_.get(); }
private:
std::unique_ptr<B> b_;
};
int main()
{
std::unique_ptr<B> b;
if(false)
b = make_unique<B1>();
else
b = make_unique<B2>();
A a(std::move(b));
a.getB()->print();
}