Notes on Polymorphism in C++

abstract.cpp

class Foo {
public:
    // A purely virtual destructor will prevent this class being instantiated directly
    virtual ~Foo() = 0;
};

class Bar: public Foo {
    ~Bar() override {  };
};

engine.cpp

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

using namespace std;

// Abstract class that declares a virtual destructor
class Engine {
public:
    virtual ~Engine() = 0;

    string get_name() { return name; };


protected:
    string name;
};

Engine::~Engine() {
    cout << "Engine (" << name << ") destructed" << endl;
}

class M123 : public Engine {
public:
    explicit M123(string name) {
        this->name = move(name);
    }

    ~M123() override { cout << name << " destructed (M123)" << endl; }

};

class M456 : public Engine {
public:
    explicit M456(string name) {
        this->name = move(name);
    }

    ~M456() override { cout << name << " destructed (M456)" << endl; }
};


void do_something() {
    cout << ">> The following vector (v1) needs explicit deletion" << endl;
    auto v1 = vector<Engine *>();
    v1.push_back(new M123("e123"));
    v1.push_back(new M456("e456"));
    for (auto &e : v1) cout << e->get_name() << endl;
    for (auto e : v1) delete (e);                        // <- Explicit destructor calling

    cout << endl << ">> The following vector (v2) will be cleaned up when the scope if left" << endl;
    auto v2 = vector<std::unique_ptr<Engine>>();
    v2.push_back(std::make_unique<M123>("f123"));
    v2.push_back(std::make_unique<M456>("f456"));
    for (auto &i : v2) cout << i->get_name() << endl;
    cout << ">> ending scope" << endl;
}

int main() {
    do_something();
}

engine_output.txt

>> The following vector (v1) needs explicit deletion
e123
e456
e123 destructed (M123)
Engine (e123) destructed
e456 destructed (M456)
Engine (e456) destructed

>> The following vector (v2) will be cleaned up when the scope is left
f123
f456
>> ending scope
f123 destructed (M123)
Engine (f123) destructed
f456 destructed (M456)
Engine (f456) destructed

inheritance.cpp

class Foo {
};

class Bar: public Foo {
};