Default constructor that calls peer constructor with unique_ptr move

Question

I am trying to make a class with two constructors. One that is a default constructor, the other calling the parameterized constructor. I get a compiler error that tells me that I cannot use move on the object just created and I sort of understand that it doesn't like to do that, because there is no real assignment here.

How can I achieve the right behavior? I am trying to avoid writing two constructors that initialize the variables. An initialization function might work, but then I would have to fill the body of the constructors and I was trying to come up with a neat solution like shown below.

#include <string>
#include <iostream>
#include <memory>

using namespace std;

class Foo
{
public:
    Foo(unique_ptr<int>& number) : m_number(move(number))
    {

    }

    Foo() : Foo(make_unique<int>(54))
    {

    }

    void print()
    {
        cout << m_number << endl;
    }

private:
    unique_ptr<int> m_number;
};

int main()
{
    Foo f;
    f.print();

    return 0;
}

main.cpp:18:33: error: invalid initialization of non-const reference of type ‘std::unique_ptr&’ from an rvalue of type ‘std::_MakeUniq::__single_object {aka std::unique_ptr}’ Foo() : Foo(make_unique(54))

Answer 1

I decided to go for an rvalue constructor. This seems to resolve the issue for me.

#include <string>
#include <iostream>
#include <memory>

using namespace std;

class Foo
{
public:
    // rvalue constructor so that we can move the unique_ptr.
    Foo(unique_ptr<int>&& number) : m_number(move(number))
    {

    }

    Foo() : Foo(make_unique<int>(54))
    {

    }

    void print()
    {
        cout << *m_number << endl;
    }

private:
    unique_ptr<int> m_number;
};

int main()
{
    Foo f;
    f.print();

    unique_ptr<int> a = make_unique<int>(33);
    Foo f2(move(a)); // important to do a move here, because we need an rvalue.
    f2.print();


    return 0;
}