---
tags: c++, c++11, CRTP
---
# Curiously Recurring Template Pattern，從入門到放棄
<!-- .slide: data-transition="fade-out" -->
---
## Polymorphism
一個介面，不同行為
<!-- .slide: data-transition="fade-out" -->
---
## Dynamic Polymorphism
Virtual function
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
class Shape {
public:
virtual ~Shape() = default;
virtual void Draw() = 0;
};
```
```cpp=+
class Circle : public Shape {
public:
~Circle() = default;
void Draw() override {
std::cout << "Circle::Draw" << std::endl;
}
};
```
```cpp=+
class Square : public Shape {
public:
~Square() = default;
void Draw() override {
std::cout << "Square::Draw" << std::endl;
}
};
```
<!-- .slide: data-transition="fade-out" -->
---
## Static Polymorphism
CRTP
<!-- .slide: data-transition="fade-out" -->
----
* [Wiki](https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern)
* [C++ Now 2018](https://www.youtube.com/watch?v=0hy3DhE5pIo)
<!-- .slide: data-transition="fade-out" -->
----
> I'm really surprised this was allowed in a C++ conference. Literally the Wikipedia page for CRTP has this example in it. It has been known since C++98 and way before that even.
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
template <typename Derived>
class Shape {
public:
void Draw() {
static_cast<Derived*>(this)->Draw();
}
};
```
```cpp=+
class Circle : public Shape<Circle> {
public:
void Draw() {
std::cout << "Circle::Draw" << std::endl;
}
};
```
```cpp=+
class Square : public Shape<Square> {
public:
void Draw() {
std::cout << "Square::Draw" << std::endl;
}
};
```
<!-- .slide: data-transition="fade-out" -->
---
## Static VS. Dynamic
* 效能
* 可維護性
* 可讀性
<!-- .slide: data-transition="fade-out" -->
---
## Non-void function (Cont'd)
```cpp=
template <typename Derived>
struct Shape {
bool Draw() {
return static_cast<Derived*>(this)->Draw();
}
};
```
```cpp=+
struct Circle : public Shape<Circle> {
bool Draw();
};
```
```cpp=+
Circle c;
c.Draw();
```
<!-- .slide: data-transition="fade-out" -->
----
## Non-void function
```cpp=
template <typename Derived>
struct Shape {
bool Draw() {
return static_cast<Derived*>(this)->Draw();
}
};
```
```cpp=+
struct Circle : public Shape<Circle> {
//bool Draw();
};
```
```cpp=+
Circle c;
c.Draw();
```
<!-- .slide: data-transition="fade-out" -->
----
## Since C++14
```cpp=
template <typename Derived>
struct Shape {
auto Draw() {
return static_cast<Derived*>(this)->Draw();
}
};
```
```cpp=+
struct Circle : public Shape<Circle> {
//bool Draw();
};
```
```cpp=+
// complie error
Circle c;
c.Draw();
```
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
template <typename Derived>
struct Shape {
auto Draw() {
return static_cast<Derived*>(this)->DoDraw();
}
};
```
```cpp=+
struct Circle : public Shape<Circle> {
bool DoDraw();
};
```
```cpp=+
struct Square : public Shape<Square> {
void DoDraw();
};
```
```cpp=+
Circle c;
c.Draw();
Square s;
s.Draw();
```
<!-- .slide: data-transition="fade-out" -->
----
## Polymorphism or not
```cpp=
template <typename Derived>
struct Shape {
template<typename ...Args>
auto Draw(Args&& ...args) {
return self()->DoDraw(std::forward<Args>(args)...);
}
Derived* self() {
return static_cast<Derived*>(this);
}
};
```
```cpp=+
struct Circle : public Shape<Circle> {
bool DoDraw(int radius);
};
```
```cpp=+
struct Square : public Shape<Square> {
void DoDraw(int width, int height);
};
```
<!-- .slide: data-transition="fade-out" -->
---
## Modern CRTP
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
template <typename Derived>
class Shape {
public:
auto Draw() const {
return self().DoDraw();
}
```
```cpp=+
private:
// isolate the static_cast
inline Derived& self() {
return *static_cast<Derived*>(this);
}
inline Derived const& self() const {
return *static_cast<const Derived*>(this);
}
};
```
<!-- .slide: data-transition="fade-out" -->
----
```cpp=+
class Circle final : public Shape<Circle> {
private:
auto DoDraw() const;
friend class Shape<Circle>;
};
```
<!-- .slide: data-transition="fade-out" -->
---
## Container
<!-- .slide: data-transition="fade-out" -->
----
## std::vector<????????>
1. Shape
2. Circle
3. Square
4. Shape<Circle>
5. Shape<Square>
<!-- .slide: data-transition="fade-out" -->
----
<!-- .slide: data-transition="fade-out" -->
---
## std::variant
> The class template std::variant represents a `type-safe union`.
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
using var_t = std::variant<Circle, Square>;
std::vector<var_t> v{Circle{}, Square{}};
try {
auto shape1 = std::get<Circle>(v[0]);
auto shape2 = std::get<Square>(v[1]);
}
catch(std::bad_variant_access&) {
}
```
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
auto* p = std::get_if<Circle>(&v[0]);
if (p != nullptr)
...
else
...
```
<!-- .slide: data-transition="fade-out" -->
----
## std::visit
```cpp=
struct Visitor {
void operator()(const Circle& s) { s.Draw(); }
void operator()(const Square& s) { s.Draw(); }
};
```
```cpp=+
int main() {
using var_t = std::variant<Circle, Square>;
std::vector<var_t> v{Circle{}, Square{}};
Visitor visitor;
std::for_each(std::cbegin(v), std::cend(v)
, [&visitor](const auto& s){
std::visit(visitor, s);
});
return 0;
}
```
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
std::for_each(std::cbegin(v), std::cend(v)
, [](const auto& s) {
std::visit([](const auto& shape){ shape.Draw();}, s);
});
```
<!-- .slide: data-transition="fade-out" -->
----
```cpp=
template<typename... Ts>
struct overloaded : Ts... { using Ts::operator()...; };
template<typename... Ts>
overloaded(Ts...) -> overloaded<Ts...>;
```
```cpp=+
std::for_each(std::cbegin(v), std::cend(v)
, [](const auto& s) {
std::visit(overloaded {
[](const Circle& shape){ shape.Draw(); },
[](const Square& shape){ shape.Draw(); },
} , s);
});
```
<!-- .slide: data-transition="fade-out" -->
---
## virtual function VS. CRTP
效能沒有很明顯的差異
* 換compiler (g++、clang++)
* Optimization level (O0~O3)
* inline
<!-- .slide: data-transition="fade-out" -->
---
## Conclusion
* 用virtual function就能完成絕大部分需要polymorphism的需求
* 設計library時可以考慮使用CRTP，但不一定是為了實現polymorphism
<!-- .slide: data-transition="fade-out" -->
---
# Q & A
<!-- .slide: data-transition="fade-out" -->
Sign in with Wallet
Connect another wallet