CRTP

Curiously Recurring Template Pattern

形式：将子类作为父类的模板参数传入。

template<typename T>
class Base {
public:
    // instead of using a virtual function, use: 
    void foo() const {
        static_cast<const T*>(this)->foo();
    }
}

// pass Derived as the T in Base!
class Derived: public Base<Derived> {
public:
    void foo() const {
        cout << "I'm derived." << endl;
    }
}

目的：代替动态绑定的虚函数，加速多态中子类函数的调用。同时为了保留多态性，通常使用如下形式：

#include <iostream>
#include <vector>

using std::cout; using std::endl;
using std::vector;

class Animal {
 public:
    virtual void say () const = 0;
    virtual ~Animal() {}
};

template <typename T>
class Animal_CRTP: public Animal {
 public:
    void say() const override {
        static_cast<const T*>(this)->say();
    }
};

class Cat: public Animal_CRTP<Cat> {
 public:
    void say() const {
        cout << "Meow~ I'm a cat." << endl;
    }
};

class Dog: public Animal_CRTP<Dog> {
 public:
    void say() const {
        cout << "Wang~ I'm a dog." << endl;
    }
};

int main () {
    vector<Animal*> zoo;
    zoo.push_back(new Cat());
    zoo.push_back(new Dog());
    for (vector<Animal*>::const_iterator iter{zoo.begin()}; iter != zoo.end(); ++iter) {
        (*iter)->say();
    }
    for (vector<Animal*>::iterator iter{zoo.begin()}; iter != zoo.end(); ++iter) {
        delete (*iter);
    }
    return 0;
}