From Sean Parent's presentation at GN13. Making a polymorphic document container with undo support.

polymorph.cpp

//
// From GoingNative 2013 "Inheritance is the Base Class of Evil" talk.
//
#include <memory>
#include <iostream>
#include <vector>
#include <cassert>

using namespace std;

template <typename T>
void draw(const T& x, ostream& out, size_t position)
{ out << string(position, ' ') << x << endl; }

class object_t
{
public:
    template <typename T>
    object_t(T x) : self_(make_shared<model<T>>(move(x))) {}

    friend void draw(const object_t& x, ostream& out, size_t position)
    { x.self_->draw_(out, position); }

private:
    struct concept_t {
        virtual ~concept_t() = default;
        virtual void draw_(ostream& out, size_t position) const = 0;
    };
    template <typename T>
    struct model : concept_t {
        model(T x) : data_(move(x)) {}
        void draw_(ostream& out, size_t position) const override
        { draw(data_, out, position); }

        T data_;
    };

    shared_ptr<const concept_t> self_;
    // A shared_ptr to a const object has value semantics.
    // Copy-on-write can be obtained using shared_ptr::unique()
};

using document_t = vector<object_t>;

void draw(const document_t& x, ostream& out, size_t position)
{
    out << string(position, ' ') << "<document>" << endl;
    for (const auto& e : x) draw(e, out, position+2);
    out << string(position, ' ') << "</document>" << endl;
}

using history_t = vector<document_t>;

void commit(history_t& x) { assert(x.size()); x.push_back(x.back()); }
void undo(history_t& x) { assert(x.size()); x.pop_back(); }
document_t& current(history_t& x) { assert(x.size()); return x.back(); }

class my_class_t {
public:
    my_class_t() = default;
};

void draw(const my_class_t& x, ostream& out, size_t position)
{
    out << string(position, ' ') << "my_class_t" << endl;
}

int main()
{
    history_t h(1);

    current(h).emplace_back(0);
    current(h).emplace_back(string("Hello"));

    draw(current(h), cout, 0);
    cout << "----------------------------------" << endl;

    commit(h);

    current(h).emplace_back(current(h));
    current(h).emplace_back(my_class_t());
    current(h)[1] = string("World");

    draw(current(h), cout, 0);
    cout << "----------------------------------" << endl;

    undo(h);

    draw(current(h), cout, 0);
}

Output:

<document>
  0
  Hello
</document>
----------------------------------
<document>
  0
  World
  <document>
    0
    Hello
  </document>
  my_class_t
</document>
----------------------------------
<document>
  0
  Hello
</document>

To enable compilation on visual studio #include should be added