What's wrong with my move statement wrt unique_ptr

[rawfool]

Hello,

I'm trying to learn unique_ptr, so I'm playing with it to get some idea.

I tried to write a function that returns a unique_ptr; called the function in main(). But my code produces an error, which I'm not able to understand. Kindly help me in resolving this.

#include <iostream>
 
#include <memory>
 
using namespace std;
 
class String
{
    string d_string;
public:
    String() { cout << "Default ctor" << endl; }
    String(string const & obj) : d_string(obj) { cout << "Parameterized Ctor" << endl; }
    String(String && tmp) : d_string(tmp.d_string)
    {
        tmp.d_string = nullptr;
        cout << "Move Ctor" << endl;
    }
    String(String const & obj) : d_string(obj.d_string)
    {
        cout << "Copy Ctor" << endl;
    }
    ~String() { cout << "Dtor" << endl; }    
    friend ostream & operator<<(ostream & out, unique_ptr<String> & obj);
    String & operator=(String const & rhs)
    {
        d_string = rhs.d_string;
        return *this;
    }
    void setVal(string const & str)
    {
        d_string = str;
    }
 
    string getVal() const { return d_string; }
};
 
// I haven't used this as it was reporting error.
// So added a getVal() member function 
ostream & operator<<(ostream & out, unique_ptr<String> & obj)
{
    out << obj->d_string << endl;
    return out;
}
 
unique_ptr<string> && f()
{
    unique_ptr<String> p;
    p->setVal(string("rahul"));
    return move(p);
}
 
int main()
{
    unique_ptr<String> ptr = f();
    cout << ptr->getVal() << endl;
}

#include <iostream>

#include <memory>

using namespace std;

class String
{
    string d_string;
public:
    String() { cout << "Default ctor" << endl; }
    String(string const & obj) : d_string(obj) { cout << "Parameterized Ctor" << endl; }
    String(String && tmp) : d_string(tmp.d_string)
    {
        tmp.d_string = nullptr;
        cout << "Move Ctor" << endl;
    }
    String(String const & obj) : d_string(obj.d_string)
    {
        cout << "Copy Ctor" << endl;
    }
    ~String() { cout << "Dtor" << endl; }    
    friend ostream & operator<<(ostream & out, unique_ptr<String> & obj);
    String & operator=(String const & rhs)
    {
        d_string = rhs.d_string;
        return *this;
    }
    void setVal(string const & str)
    {
        d_string = str;
    }
    
    string getVal() const { return d_string; }
};

// I haven't used this as it was reporting error.
// So added a getVal() member function 
ostream & operator<<(ostream & out, unique_ptr<String> & obj)
{
    out << obj->d_string << endl;
    return out;
}

unique_ptr<string> && f()
{
    unique_ptr<String> p;
    p->setVal(string("rahul"));
    return move(p);
}

int main()
{
    unique_ptr<String> ptr = f();
    cout << ptr->getVal() << endl;
}

To copy to clipboard, switch view to plain text mode 

And the error is

main.cpp: In function ‘std::unique_ptr<std::__cxx11::basic_string<char> >&& f()’:
main.cpp:47:16: error: invalid initialization of reference of type ‘std::unique_ptr<std::__cxx11::basic_string<char> >&&’ from expression of type ‘std::remove_reference<std::unique_ptr<String>&>::type {aka std::unique_ptr<String>}’
     return move(p);
            ~~~~^~~
main.cpp: In function ‘int main()’:
main.cpp:52:31: error: conversion from ‘std::unique_ptr<std::__cxx11::basic_string<char> >’ to non-scalar type ‘std::unique_ptr<String>’ requested
     unique_ptr<String> ptr = f();
                              ~^~
make: *** [Makefile:6: main.o] Error 1

main.cpp: In function ‘std::unique_ptr<std::__cxx11::basic_string<char> >&& f()’:
main.cpp:47:16: error: invalid initialization of reference of type ‘std::unique_ptr<std::__cxx11::basic_string<char> >&&’ from expression of type ‘std::remove_reference<std::unique_ptr<String>&>::type {aka std::unique_ptr<String>}’
     return move(p);
            ~~~~^~~
main.cpp: In function ‘int main()’:
main.cpp:52:31: error: conversion from ‘std::unique_ptr<std::__cxx11::basic_string<char> >’ to non-scalar type ‘std::unique_ptr<String>’ requested
     unique_ptr<String> ptr = f();
                              ~^~
make: *** [Makefile:6: main.o] Error 1

To copy to clipboard, switch view to plain text mode 

I have no idea what's happening. Please guide me if my implementation is wrong. Thanks.

[d_stranz]

In line 45 you use "string", in line 47 you use "String". The types are different and not convertible.