C++单例实现实践

简介

• 保证一个class只有一个实体，并为它提供一个全局访问点。
  

• 种经过改进的全局变量。
  

懒汉模式 版本一

class Singleton 
{
public :
	static Singleton* instance() 
	{
		if (!pInstance_)
			pInstance = new Singleton();
		return pInstance_;
	}
private :
	Singleton(); 		
	Singleton(const Singleton&);
	~Singleton();
private:
	static Singleton* pInstance_;
};

Singleton* Singleton::pInstance_ = nullptr;

版本二

class Singleton 
{
public :
	static Singleton* instance() 
	{
		if (!pInstance_)
			pInstance = new Singleton();
		return pInstance_;
	}
	
	Singleton() = delete;	
	Singleton(const Singleton&) = delete;
	Singleton(Singleton&&) = delete;
	Singleton& operator=(const Singleton&) = delete;
	Singleton& operator=(Singleton&&) = delete;
	~Singleton() = delete;
	
private:
	static Singleton* pInstance_;
};

Singleton* Singleton::pInstance_ = nullptr;

版本三

• 线程安全

class Singleton 
{
public :
	static Singleton* instance() 
	{
		std::lock_gurad lg(mt);
		if (!pInstance_)
			pInstance = new Singleton();
		return pInstance_;
	}
	
	Singleton() = delete;	
	Singleton(const Singleton&) = delete;
	Singleton(Singleton&&) = delete;
	Singleton& operator=(const Singleton&) = delete;
	Singleton& operator=(Singleton&&) = delete;
	~Singleton() = delete;
	
private:
	static Singleton* pInstance_;
	static std::mutex mt;
};

Singleton* Singleton::pInstance_ = nullptr;
std::mutex Singleton::mt;

• 双检测锁定

Singleton* Singleton::instance() 
{
	if (!pInstance_)
	{
		std::lock_gurad lg(mt);
		if (!pInstance_)
			pInstance = new Singleton();
	}
	return pInstance_;
}

版本四

class Singleton 
{
public :
	static Singleton& instance() 
	{
		static Singleton inst;
		return inst;
	}
	
	Singleton() = delete;	
	Singleton(const Singleton&) = delete;
	Singleton(Singleton&&) = delete;
	Singleton& operator=(const Singleton&) = delete;
	Singleton& operator=(Singleton&&) = delete;
	~Singleton() = delete;
	
private:
	// ...
};

饿汉模式

class Singleton 
{
public :
	static Singleton& instance() 
	{
		return inst;
	}
	
	Singleton() = delete;	
	Singleton(const Singleton&) = delete;
	Singleton(Singleton&&) = delete;
	Singleton& operator=(const Singleton&) = delete;
	Singleton& operator=(Singleton&&) = delete;
	~Singleton() = delete;
	
private:
	static Singleton inst;
};

Singleton Singleton::inst;

避免资源泄露

Singleton构造函数可以（可能）索求广泛的资源：网络连接、OS互斥体和进程通讯（IPC)方法中的各种 handles、进程外的CORBA或 COM对象的reference，等等。

有效手段：第一次执行才初始化

Singleton& Singleton::instance()
{
	static Singleton obj; // static变量，第一次执行才初始化
	return obj ;
}

优先单例

class Singleton 
{
public :
	static Singleton& instance() 
	{
		static Singleton inst;
		return inst;
	}
	
	Singleton() = delete;	
	Singleton(const Singleton&) = delete;
	Singleton(Singleton&&) = delete;
	Singleton& operator=(const Singleton&) = delete;
	Singleton& operator=(Singleton&&) = delete;
	~Singleton() = delete;
	
private:
	// ...
};

优点：

• 无线程安全问题
• 懒汉模式，只要被使用到时才创建对象
• 局部静态变量，能保证资源得到回收