Set callback function using unique_ptr of another class

I am in need to implement a callback method using a unique_ptr of another class:

#include <iostream>
#include <functional>
#include <memory>

class A
{
public:
    void Show(void) {}
};

class B
{
public:
    void SetCB(std::function<void(void)> callb);
private:
    std::function<void(void)> cb;
};

void B::SetCB(std::function<void(void)> callb)
{
    cb= callb;
}

int main()
{
    std::unique_ptr<A> a1 = std::make_unique<A>();
    std::unique_ptr<B> b1 = std::make_unique<B>();
    b1->SetCB(&a1->Show, a1.get());
}

I am getting compilation error:

$ c++ -std=c++14 try68.cpp
try68.cpp: In function 'int main()':
try68.cpp:28:29: error: no matching function for call to 'B::SetCB(<unresolved overloaded function type>, std::unique_ptr<A>::pointer)'
 b1->SetCB(a1->Show, a1.get());
                             ^
try68.cpp:28:29: note: candidate is:
try68.cpp:19:6: note: void B::SetCB(std::function<void()>)
 void B::SetCB(std::function<void(void)> callb)
      ^
try68.cpp:19:6: note:   candidate expects 1 argument, 2 provided

                 ^

Is it not possible to set a callback methods using unique_ptr?