C++ unique_ptr casting from derived to base

Question

I have been reading around and am struggling to understand and get this working.

I have a base class Person, which Teacher & Student inherit from. I want to store them both inside of a vector of type "Person". I have tried a few things. However, I keep getting pages of errors, and I am struggling to understand them.

This current code compiled with g++ -std=c++17 test.cpp is giving me:

Undefined symbols for architecture x86_64:
  "Person::~Person()", referenced from:
      Teacher::~Teacher() in test-9423bf.o
      Student::~Student() in test-9423bf.o
ld: symbol(s) not found for architecture x86_64

Would appreciate any tips and good references on c++ features written simply.

#include <iostream>
#include <vector>
#include <memory>

class Person {
public:
    virtual void printName() = 0;
    virtual ~Person() = 0;
};

class Teacher : public Person {
public:
    void printName() {
        std::cout << "Hello My Name is Teacher" << std::endl;
    }
    ~Teacher() {}

};

class Student : public Person {
public:
    void printName() {
        std::cout << "Hello My Name Is Student" << std::endl;
    }

    ~Student() {}

};

//Capturing the raw pointer and letting it go out of scope
template<typename Person, typename Teacher>
std::unique_ptr<Person> static_unique_pointer_cast (std::unique_ptr<Teacher>&& old){
    return std::unique_ptr<Person>{static_cast<Person*>(old.release())};
    //conversion: unique_ptr<FROM>->FROM*->TO*->unique_ptr<TO>
}

auto main() -> int {


    auto t1 = std::make_unique<Teacher>();
    auto t2 = std::make_unique<Teacher>();
    auto t3 = std::make_unique<Teacher>();

    auto s1 = std::make_unique<Student>();
    auto s2 = std::make_unique<Student>();
    auto s3 = std::make_unique<Student>();

    std::vector<std::unique_ptr<Person>> v;
    // v.push_back(static_unique_pointer_cast<Person>(std::move(s1)));

    auto foo = static_unique_pointer_cast<Person>(std::move(s1));
    // std::vector<std::unique_ptr<Person>> ve = {
    //     std::move(t1), 
    //     std::move(t2), 
    //     std::move(t3), 
    //     std::move(s1), 
    //     std::move(s2), 
    //     std::move(s3)
    //     };

    return 0;
}

Edit: I got it working by changing base class destructor to default.

I now have this:

std::vector<std::unique_ptr<Person>> v;
v.push_back(static_unique_pointer_cast<Person>(std::move(s1)));
v.push_back(static_unique_pointer_cast<Person>(std::move(s1)));

for (auto item: v) {
    item->printName();
}

but I am getting the following error:

error: call to implicitly-deleted copy constructor of 'std::__1::unique_ptr<Person, std::__1::default_delete<Person> >'
    for (auto item: v) {

edit 2:

The above works when I use:

for (auto &&item: v) {
        item->printName();
    }

Could someone explain this to me? The vector holds unique pointers (that were once an rvalue (specifically exrvalue) but now they aren't. Why do I need to use auto &&?

Answer 1

I've cleaned up the code an will try to explain the changes.

#include <iostream>
#include <vector>
#include <memory>

class Person {
public:
    virtual ~Person() = default; // before this was a pure-virtual d'tor. Usually, you don't need it, but the linker told you it wasn't implemented

    virtual void printName() = 0;
};

class Teacher : public Person {
public:
    void printName() override {
        std::cout << "Hello My Name is Teacher\n";
    }
    // removed d'tor since you already have a default virtual destructor here
};

class Student : public Person {
public:
    void printName() override {
        std::cout << "Hello My Name Is Student\n";
    }
    // removed d'tor since you already have a default virtual destructor here
};

// removed template this upcasting is almost straight forward.

auto main() -> int {

    auto t1 = std::make_unique<Teacher>();
    auto t2 = std::make_unique<Teacher>();
    auto t3 = std::make_unique<Teacher>();

    auto s1 = std::make_unique<Student>();
    auto s2 = std::make_unique<Student>();
    auto s3 = std::make_unique<Student>();

    std::vector<std::unique_ptr<Person>> v;
    v.push_back(std::move(t1));
    v.push_back(std::move(t2));
    v.push_back(std::move(t3)); // Easy to add Students and Teachers
    v.push_back(std::move(s1));
    v.push_back(std::move(s2));
    v.push_back(std::move(s3));

    for (auto &item: v) { // Taking a reference, `&`, the pointers in `v` still stay there after the loop and can be reused. Don't use `&&` unless you want the use the pointers once only.
        item->printName();
    }

    return 0;
}