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From Sean Parent's presentation at GN13. Making a polymorphic document container with undo support.
//
// 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);
}
@berkus

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Owner

berkus commented Oct 18, 2013

Output:

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

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mrboojum commented Sep 17, 2016

To enable compilation on visual studio #include should be added

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