C++深入详解单例模式与特殊类设计的实现

#include <iostream>
using namespace std;
class Singleton
{
public:
	static Singleton& GetInstance()
	{
		return _instance;
	}
	int GetRandom()//由于侧重点不是随机数 所以直接返回一个30
	{
		return 30;
	}
private:
	Singleton() {}//构造函数私有
	Singleton(Singleton&) = delete;//防拷贝，被=delete修饰表明这个函数被删除，即可以只声明不实现,换言之禁用了该函数
	Singleton& operator=(const Singleton&) = delete;//防拷贝
	static Singleton _instance;
};
Singleton Singleton::_instance;//类外必须初始化，类内只是声明
int main()
{
	//1.调用
	cout<<Singleton::GetInstance().GetRandom()<<endl;
	//2.
	Singleton& s = Singleton::GetInstance();
	cout<<s.GetRandom()<<endl;
	return 0;
}

Singleton s;//err
Singleton s1(s);//err
Singleton s1=s;//err

class Singleton
{
public:
	static Singleton* GetInstance()
	{
		if (_instance == nullptr)
		{
			_mtx.lock();//double lock保证线程安全
			if (_instance == nullptr)//必须再次检查  不然可能另一个线程那已经new完了，这边又new违背了单例，且可能覆盖数据。
			{
				_instance = new Singleton();
			}
			_mtx.unlock();
		}
		return _instance;
	}
	int GetRandom()
	{
		return 30;
	}
	class Clear//资源回收的内部类，必须是公有的，不然外部声明报错
	{
	public:
		~Clear()
		{
			cout << "释放资源" << endl;
			delete _instance;
		}
	};
private:
	static Clear _cle;
	Singleton() {}
	Singleton(Singleton&) = delete;
	Singleton& operator=(const Singleton&) = delete;
	static Singleton* _instance;
	static mutex _mtx;//线程安全
};
Singleton* Singleton::_instance = nullptr;
mutex Singleton::_mtx;
Singleton::Clear _cle;
int main()
{	
	//1.
	cout<<Singleton::GetInstance()->GetRandom()<<endl;
	//2/
	static Singleton* s = Singleton::GetInstance();
	cout << s->GetRandom() << endl;
	//不能用左值引用接收的原因  返回值是右值 得用右值引用接收
	//为什么函数返回值是右值  因为返回值是借助临时变量返回的拿不到地址
	//如果返回值是左值引用就能用左值引用接收
	return 0;
}

class HeapOnly
{
public:
	static HeapOnly* GetInstance()
	{
		return new HeapOnly;
	}
	void Test()
	{
		cout << "I am Test" << endl;
	}
private:
	HeapOnly() {}
	HeapOnly(HeapOnly&) = delete;
	HeapOnly& operator=(const HeapOnly&) = delete;
};
int main()
{	
	HeapOnly* ho = HeapOnly::GetInstance();
	ho->Test();
	delete ho;
	return 0;
}

class HeapOnly
{
public:
	void Test()
	{
		cout << "I am Test" << endl;
	}
private:
	~HeapOnly();
};
int main()
{	
	//HeapOnly ho_stack;//err
	HeapOnly* ho = new HeapOnly;
	ho->Test();
	return 0;
}