Question

I understand the reason for the error C2662 indicated below. My question is why the call
a.GetB()->Get() in main() doesn’t incur in a similar error, as GetB() also returns a const reference to a unique_ptr<B> object ?

#include <iostream>
#include <memory>

using namespace std;

class B
{
    int i;

    public:
    B(int j) : i(j) {}
    int Get() { return i; }
};

class A
{
    std::unique_ptr<B> uptrB;

    public:
    A(int i) : uptrB(new B(i)) {}
    const std::unique_ptr<B>& GetB() { return uptrB; }
    const B* GetB1() { return uptrB.get(); }
};

int main()
{
    A a(3);
    cout << a.GetB()->Get() << endl;
    cout << a.GetB1()->Get() << endl;  // error C2662:'B::Get' cannot conver 'this' pointer from 'const B' to 'B&'
}

Answer 1

const std::unique_ptr<B> is analogous to B* const, that is, an immutable pointer to a mutable B—not const B*, a mutable pointer to an immutable B. If you wanted to get the same error from the unique_ptr version, you would need to write std::unique_ptr<const B>. As it is, you are returning the unique_ptr by const reference, but the B to which it refers is not const.