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crtp_modern.cpp(file created)
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| 1 | + | // crtp_modern.cpp — compile with: c++ -std=c++20 -O2 crtp_modern.cpp | |
| 2 | + | // | |
| 3 | + | // CRTP (Curiously Recurring Template Pattern) shines when a base class needs | |
| 4 | + | // to return or operate on the *derived* type without paying for virtual | |
| 5 | + | // dispatch. Two practical mixins below. | |
| 6 | + | ||
| 7 | + | #include <concepts> | |
| 8 | + | #include <iostream> | |
| 9 | + | #include <string> | |
| 10 | + | #include <utility> | |
| 11 | + | ||
| 12 | + | // --------------------------------------------------------------------------- | |
| 13 | + | // 1. Fluent-builder mixin | |
| 14 | + | // | |
| 15 | + | // Problem: chained setters in a base class return `Base&`, which loses the | |
| 16 | + | // derived type and breaks further chained calls to derived methods. | |
| 17 | + | // CRTP fix: the base knows the derived type and casts `*this` to it. | |
| 18 | + | // --------------------------------------------------------------------------- | |
| 19 | + | template <class Derived> | |
| 20 | + | class FluentBase { | |
| 21 | + | public: | |
| 22 | + | Derived& self() noexcept { return static_cast<Derived&>(*this); } | |
| 23 | + | ||
| 24 | + | Derived& named(std::string n) & { | |
| 25 | + | name_ = std::move(n); | |
| 26 | + | return self(); | |
| 27 | + | } | |
| 28 | + | ||
| 29 | + | const std::string& name() const noexcept { return name_; } | |
| 30 | + | ||
| 31 | + | private: | |
| 32 | + | std::string name_; | |
| 33 | + | }; | |
| 34 | + | ||
| 35 | + | class HttpRequest : public FluentBase<HttpRequest> { | |
| 36 | + | public: | |
| 37 | + | HttpRequest& method(std::string m) { method_ = std::move(m); return *this; } | |
| 38 | + | HttpRequest& url(std::string u) { url_ = std::move(u); return *this; } | |
| 39 | + | ||
| 40 | + | void send() const { | |
| 41 | + | std::cout << '[' << name() << "] " << method_ << ' ' << url_ << '\n'; | |
| 42 | + | } | |
| 43 | + | ||
| 44 | + | private: | |
| 45 | + | std::string method_, url_; | |
| 46 | + | }; | |
| 47 | + | ||
| 48 | + | // --------------------------------------------------------------------------- | |
| 49 | + | // 2. Arithmetic mixin | |
| 50 | + | // | |
| 51 | + | // Define `+=`, `-=`, `*=` in your type; the mixin synthesizes `+`, `-`, `*` | |
| 52 | + | // returning the derived type. No virtuals, no duplicated boilerplate per type. | |
| 53 | + | // --------------------------------------------------------------------------- | |
| 54 | + | template <class Derived> | |
| 55 | + | struct Arithmetic { | |
| 56 | + | friend Derived operator+(Derived a, const Derived& b) { a += b; return a; } | |
| 57 | + | friend Derived operator-(Derived a, const Derived& b) { a -= b; return a; } | |
| 58 | + | friend Derived operator*(Derived a, const Derived& b) { a *= b; return a; } | |
| 59 | + | }; | |
| 60 | + | ||
| 61 | + | struct Vec2 : Arithmetic<Vec2> { | |
| 62 | + | double x{}, y{}; | |
| 63 | + | ||
| 64 | + | Vec2& operator+=(const Vec2& o) { x += o.x; y += o.y; return *this; } | |
| 65 | + | Vec2& operator-=(const Vec2& o) { x -= o.x; y -= o.y; return *this; } | |
| 66 | + | Vec2& operator*=(const Vec2& o) { x *= o.x; y *= o.y; return *this; } | |
| 67 | + | ||
| 68 | + | friend std::ostream& operator<<(std::ostream& os, const Vec2& v) { | |
| 69 | + | return os << '(' << v.x << ", " << v.y << ')'; | |
| 70 | + | } | |
| 71 | + | }; | |
| 72 | + | ||
| 73 | + | // --------------------------------------------------------------------------- | |
| 74 | + | // Demo | |
| 75 | + | // --------------------------------------------------------------------------- | |
| 76 | + | int main() { | |
| 77 | + | HttpRequest{} | |
| 78 | + | .named("health-check") // FluentBase method, returns HttpRequest& | |
| 79 | + | .method("GET") // derived method still reachable | |
| 80 | + | .url("https://example.com/health") | |
| 81 | + | .send(); | |
| 82 | + | ||
| 83 | + | Vec2 a{1.0, 2.0}, b{3.0, 4.0}; | |
| 84 | + | std::cout << a + b << " | " << a * b << '\n'; | |
| 85 | + | } | |
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