class Singleton{
    Singleton() { }
    static Singleton* m_instance_ptr;
public:
    static Singleton* get_instance() {
        return m_instance_ptr;
	}
};
Singleton* Singleton::m_instance_ptr = new Singleton();
int main(){
    Singleton* instance1 = Singleton::get_instance();
    Singleton* instance2 = Singleton::get_instance();
    return 0;
}

懶漢模式

延遲加載，節(jié)省資源，效率低，存在線程安全問題。

class Singleton {
    Singleton() { }
    static Singleton* m_instance_ptr;
public:
    static Singleton* get_instance() {
        if(m_instance_ptr == nullptr)
			m_instance_ptr = new Singleton();
        return m_instance_ptr;
    }
};
Singleton* Singleton::m_instance_ptr = nullptr;
int main(){
    Singleton* instance1 = Singleton::get_instance();
    Singleton* instance2 = Singleton::get_instance();
    return 0;
}

鎖 + 智能指針

線程安全（鎖）+ 內存回收（智能指針）

#include <iostream>
#include <memory>
#include <mutex>
class Singleton {
public:
	typedef std::shared_ptr<Singleton> Ptr;
    static Ptr get_instance() {
        if(m_instance_ptr == nullptr) {
            std::lock_guard<std::mutex> lk(m_mutex);
            if(m_instance_ptr == nullptr)
				m_instance_ptr = std::shared_ptr<Singleton>(new Singleton);
        }
        return m_instance_ptr;
    }
private:
	Singleton() {}
    static Ptr m_instance_ptr;
    static std::mutex m_mutex;
};
Singleton::Ptr Singleton::m_instance_ptr = nullptr;
std::mutex Singleton::m_mutex;
int main(){
    Singleton::Ptr instance1 = Singleton::get_instance();
    Singleton::Ptr instance2 = Singleton::get_instance();
    return 0;
}

局部靜態(tài)變量

class Singleton {
public:
    Singleton(const Singleton&)=delete;
    static Singleton& get_instance() {
        static Singleton instance;
        return instance;
    }
private:
    Singleton() {}
};
int main() {
	Singleton& instance1 = Singleton::get_instance();
    Singleton& instance2 = Singleton::get_instance();
    return 0;
}