CCRTP奇异递归模板奇异递归模板模式CRTP是派生类将自己作为模板参数传递给基类的技术。CRTP实现静态多态避免了虚函数的运行时开销。CRTP的基本形式基类模板接受派生类作为参数。#include#includetemplateclass Base {public:void interface() {static_cast(this)-implementation();}static void static_interface() {Derived::static_implementation();}Derived derived() {return static_cast(*this);}const Derived derived() const {return static_cast(*this);}};class DerivedA : public Base {public:void implementation() {std::cout DerivedA implementation\n;}static void static_implementation() {std::cout DerivedA static\n;}};class DerivedB : public Base {public:void implementation() {std::cout DerivedB implementation\n;}static void static_implementation() {std::cout DerivedB static\n;}};void crtp_basic() {DerivedA a;DerivedB b;a.interface();b.interface();Base::static_interface();Base::static_interface();}CRTP实现对象计数。templateclass ObjectCounter {static inline int count_ 0;static inline int alive_ 0;protected:ObjectCounter() {count_;alive_;}~ObjectCounter() {--alive_;}public:static int total_created() { return count_; }static int currently_alive() { return alive_; }};class Widget : public ObjectCounter {int id_;public:Widget(int id) : id_(id) {}int id() const { return id_; }};void object_counter_demo() {std::cout Initial: Widget::currently_alive() \n;{Widget w1(1);Widget w2(2);std::cout Two created: Widget::currently_alive() \n;std::cout Total created: Widget::total_created() \n;}std::cout After scope: Widget::currently_alive() \n;}CRTP实现比较运算符。templateclass EqualComparable {public:friend bool operator(const Derived a, const Derived b) {return a.equal_to(b);}friend bool operator!(const Derived a, const Derived b) {return !a.equal_to(b);}};templateclass Comparable : public EqualComparable {public:friend bool operator(const Derived a, const Derived b) {return a.less_than(b);}friend bool operator(const Derived a, const Derived b) {return b.less_than(a);}friend bool operator(const Derived a, const Derived b) {return !b.less_than(a);}friend bool operator(const Derived a, const Derived b) {return !a.less_than(b);}};class Point : public Comparable {int x_, y_;public:Point(int x, int y) : x_(x), y_(y) {}bool equal_to(const Point other) const {return x_ other.x_ y_ other.y_;}bool less_than(const Point other) const {if (x_ ! other.x_) return x_ other.x_;return y_ other.y_;}friend std::ostream operator(std::ostream os, const Point p) {os ( p.x_ , p.y_ );return os;}};void comparable_demo() {Point p1(1, 2), p2(1, 2), p3(3, 4);std::cout p1 p2 : (p1 p2) \n;std::cout p1 ! p3 : (p1 ! p3) \n;std::cout p1 p3 : (p1 p3) \n;std::cout p3 p1 : (p3 p1) \n;std::cout p3 p1 : (p3 p1) \n;}CRTP实现单例。templateclass Singleton {public:static T instance() {static T instance_;return instance_;}Singleton(const Singleton) delete;Singleton operator(const Singleton) delete;protected:Singleton() default;~Singleton() default;};class Logger : public Singleton {friend class Singleton;int log_count_ 0;Logger() default;public:void log(const std::string msg) {std::cout [LOG log_count_ ] msg \n;}};void singleton_crtp() {auto logger Logger::instance();logger.log(First message);logger.log(Second message);auto logger2 Logger::instance();std::cout Same instance: (logger logger2) \n;}CRTP实现克隆。templateclass Cloneable {public:std::unique_ptr clone() const {return std::unique_ptr(static_cast(this-clone_impl()));}private:virtual Cloneable* clone_impl() const 0;};class ConcreteCloneable : public Cloneable {int value_;public:explicit ConcreteCloneable(int v) : value_(v) {}ConcreteCloneable* clone_impl() const override {return new ConcreteCloneable(value_);}int value() const { return value_; }};void clone_demo() {ConcreteCloneable original(42);auto copy original.clone();std::cout Cloned value: copy-value() \n;}CRTP在C中广泛应用结合了模板的编译期效率和面向对象的表达力。