├── README.md
├── client.cpp
└── library.hpp


/README.md:
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 1 | # Better Code: Runtime Polymorphism 
 2 | 
 3 | [talk-slides]: http://sean-parent.stlab.cc/presentations/2017-01-18-runtime-polymorphism/2017-01-18-runtime-polymorphism.pdf
 4 | 
 5 | Git walk through of the C++ code presented in Sean Parent's talk [Better Code:
 6 | Runtime Polymorphism][talk-slides].
 7 | 
 8 | Casual C++ programmers or CS undergrads will be familiar with the
 9 | object-oriented code up to the "slide 13" commit. The use of the C++11 feature
10 | `std::shared_ptr` may be new and the memory leak fixed in "slide 13 (leak fix)"
11 | may cause a mild surprise. The subsequent transformations go on to show the true
12 | power of new C++ features (C++11 and C++14) and the result is _very_ client
13 | friendly code (at the price of the library containing some albeit convoluted
14 | gobbledygook C++ boilerplate).
15 | 
16 | [github-commits-page]: https://github.com/alecjacobson/better-code-runtime-polymorphism/commits/master
17 | 
18 | The [git commits][github-commits-page] of this repository are labeled with
19 | "slide x" where x corresponds to the slide number in the corresponding [talk by
20 | Sean parent][talk-slides] to moments where he compiles and runs the code.
21 | Through these commits you will see the transformations he applies. I have mostly
22 | faithfully copied his code examples, adding necessary `#include`s and `using
23 | namespace` lines to get it to compile.
24 | 
25 | Most of the time the only necessary compilation flag is C++11 support. When
26 | `unique_ptr`s are briefly used you may need C++14.
27 | 
28 | For example, on Mac OS X with Apple LLVM version 9.1.0 (clang-902.0.39.2) you
29 | can compile and run for any commit with
30 |     
31 |     clang++ -std=c++14 client.cpp && ./a.out
32 | 


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/client.cpp:
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 1 | //  clang++ -std=c++14 client.cpp -o client && ./client
 2 | #include "library.hpp"
 3 | #include <iostream>
 4 | #include <thread>
 5 | #include <future>
 6 | 
 7 | using namespace std;
 8 | 
 9 | class my_class_t {
10 |   /* ... */
11 | };
12 | 
13 | void draw(const my_class_t&, ostream& out, size_t position)
14 | { out << string(position, ' ') << "my_class_t" << endl; }
15 | 
16 | int main() {
17 |   history_t h(1);
18 |   current(h).emplace_back(0);
19 |   current(h).emplace_back(string("Hello!")); 
20 |   
21 |   draw(current(h), cout, 0);
22 |   cout << "--------------------------" << endl; 
23 | 
24 |   commit(h);
25 | 
26 |   current(h)[0] = 42.5;
27 | 
28 |   auto saving = async([document = current(h)]() {
29 |       this_thread::sleep_for(chrono::seconds(3));
30 |       cout << "-------- 'save' --------" << endl;
31 |       draw(document, cout, 0);
32 |   });
33 | 
34 |   current(h)[1] = string("World");
35 |   current(h).emplace_back(current(h));
36 |   current(h).emplace_back(my_class_t());
37 | 
38 |   draw(current(h), cout, 0);
39 |   cout << "--------------------------" << endl; 
40 | 
41 |   undo(h);
42 | 
43 |   draw(current(h), cout, 0);
44 | }
45 | 


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/library.hpp:
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 1 | #include <iostream>
 2 | #include <string>
 3 | #include <vector>
 4 | #include <cassert>
 5 | using namespace std;
 6 | 
 7 | 
 8 | template <typename T>
 9 | void draw(const T& x, ostream& out, size_t position)
10 | { out << string(position, ' ') << x << endl; }
11 | 
12 | class object_t {
13 |   public:
14 |     template <typename T>
15 |     object_t(T x) : self_(make_shared<model<T>>(move(x)))
16 |     { }
17 | 
18 |     friend void draw(const object_t& x, ostream& out, size_t position)
19 |     { x.self_->draw_(out, position); }
20 |   private:
21 |     struct concept_t {
22 |       virtual ~concept_t() = default;
23 |       virtual void draw_(ostream&, size_t) const = 0;
24 |     };
25 |     template <typename T>
26 |     struct model final : concept_t {
27 |       model(T x) : data_(move(x)) { }
28 |       void draw_(ostream& out, size_t position) const override
29 |       { draw(data_, out, position); }
30 | 
31 |       T data_;
32 |     };
33 | 
34 |     shared_ptr<const concept_t> self_;
35 | };
36 | 
37 | using document_t = vector<object_t>;
38 | 
39 | void draw(const document_t& x, ostream& out, size_t position)
40 | {
41 |     out << string(position, ' ') << "<document>" << endl;
42 |     for (const auto& e: x) draw(e, out, position + 2);
43 |     out << string(position, ' ') << "</document>" << endl;
44 | }
45 | 
46 | using history_t = vector<document_t>;
47 | 
48 | void commit(history_t& x){ assert(x.size()); x.push_back(x.back()); }
49 | void undo(history_t& x) { assert(x.size()); x.pop_back(); }
50 | document_t& current(history_t& x) { assert(x.size()); return x.back(); }
51 | 


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