【C/C++】模板类

记录一个以前没见过的模板使用方法。

#include <iostream>

template <class T>
class Derived : public T {
	public:
		Derived() { std::cout << "Derived()" << std::endl; }
		~Derived() { std::cout << "~Derived()" << std::endl; }
};

class Base {
	public:
		Base() { std::cout << "Base()" << std::endl; }
		~Base() { std::cout << "~Base()" << std::endl; }
};

int main() {
	Derived<Base> db;
	return 0;
}

输出：

Base()
Derived()
~Derived()
~Base()

用途：从未知的类Ｔ继承，然后在Ｔ的基础上附加一些必要的函数及变量。

利用这种模板函数，我们可以设计出单例。

#include <iostream>
#include <assert.h>

template <class T>
class SingletonWrapper : public T {					// 简单封装，添加一个flag
	public:
		~SingletonWrapper() { is_destroyed_ = true; }
		static bool is_destroyed_;
};

template <class T>
bool SingletonWrapper<T>::is_destroyed_ = false;

template <class T>
class Singleton {
	public:
		static T& Instance() {					// 接口函数
			static SingletonWrapper<T> t;			// 静态实例化
			assert(!SingletonWrapper<T>::is_destroyed_);
			return static_cast<T&>(t);			// 返回 T&
		}
		static bool IsDestroyed() { return SingletonWrapper<T>::is_destroyed_; }
};

class Box {
	public:
		void  Init(const uint32_t size) { 
			length_ = weigth_ = height_ = size; 
		}
		void Show() {
			std::cout << "length: " << length_ << std::endl;
			std::cout << "weigth: " << weigth_ << std::endl;
			std::cout << "height: " << height_ << std::endl;
		}
		~Box() { std::cout << "~Box()" << std::endl; }

	private:
		uint32_t length_ = 0;
		uint32_t weigth_ = 0;
		uint32_t height_ = 0;
};

class BBox : public Singleton<BBox> {				// 风骚的操作,为BBox添加了两个静态函数用于单例的构造
	public:
		void Init(const uint32_t size) { 
			length_ = weigth_ = height_ = size; 
		}
		void Show() {
			std::cout << "length: " << length_ << std::endl;
			std::cout << "weigth: " << weigth_ << std::endl;
			std::cout << "height: " << height_ << std::endl;
		}
		~BBox() { std::cout << "~BBox()" << std::endl; }

	private:
		uint32_t length_ = 0;
		uint32_t weigth_ = 0;
		uint32_t height_ = 0;
};

int main() {
	// 1
	Singleton<Box>::Instance().Init(10);
	Singleton<Box>::Instance().Show();

	// 2
	BBox::Instance().Init(20);
	BBox::Instance().Show();
	std::cout << &BBox::Instance() << std::endl;
	std::cout << &BBox::Instance().Instance().Instance() << std::endl;

	return 0;
}

输出如下：

length: 10
weigth: 10
height: 10
length: 20
weigth: 20
height: 20
0x6021d0
0x6021d0
~BBox()
~Box()

可以发现 Singleton 类用来实现单例，我们有两种使用 Singleton 模板类的方式。