c++单例模式_singleton(const singleton& other);-优快云博客

本文链接：https://blog.youkuaiyun.com/caoxin9876/article/details/109405828

这篇博客详细介绍了C++中实现单例模式的四种常见方式：恶汉式、懒汉式、线程安全的静态变量和双检测锁。每种方式都考虑了线程安全问题，通过实例展示了如何确保在多线程环境中只有一个 Singleton 实例。

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单例模式

恶汉式

#include <bits/stdc++.h>

using namespace std;

class Singleton{
private:
    Singleton() = default;
    Singleton(const Singleton& other) = default;
public:
    static Singleton* getInstance();
    static Singleton* m_instance;
    int a = 0;
};

Singleton* Singleton::m_instance = new Singleton();

// 线程安全
Singleton* Singleton::getInstance() {
    return m_instance;
}

int main()
{
    Singleton *s1 = Singleton::getInstance();
    Singleton *s2 = Singleton::getInstance();
    s1->a++;
    s2->a++;

    return 0;
}

懒汉式

#include <bits/stdc++.h>

using namespace std;

class Singleton{
private:
    Singleton() = default;
    Singleton(const Singleton& other) = default;
public:
    static Singleton* getInstance();
    static Singleton* m_instance;
    int a = 0;
};

Singleton* Singleton::m_instance=nullptr;

// 线程非安全
Singleton* Singleton::getInstance() {
    if (m_instance == nullptr) {
        m_instance = new Singleton();
    }
    return m_instance;
}

int main()
{
    Singleton *s1 = Singleton::getInstance();
    Singleton *s2 = Singleton::getInstance();
    s1->a++;
    s2->a++;

    return 0;
}

锁

#include <bits/stdc++.h>

using namespace std;

class Singleton{
private:
    Singleton() = default;
    Singleton(const Singleton& other) = default;
public:
    static Singleton* getInstance();
    static Singleton* m_instance;
    static mutex m_mutex;
    int a = 0;
};

Singleton* Singleton::m_instance=nullptr;
mutex Singleton::m_mutex;

// 线程安全
Singleton* Singleton::getInstance() {
    unique_lock<mutex> lock(m_mutex);
    if (m_instance == nullptr) {
        m_instance = new Singleton();
    }
    return m_instance;
}

int main()
{
    Singleton *s1 = Singleton::getInstance();
    Singleton *s2 = Singleton::getInstance();
    s1->a++;
    s2->a++;

    return 0;
}

双检测

using namespace std;

class Singleton{
private:
    Singleton() = default;
    Singleton(const Singleton& other) = default;
public:
    static Singleton* getInstance();
    static Singleton* m_instance;
    static mutex m_mutex;
    int a = 0;
};

Singleton* Singleton::m_instance=nullptr;
mutex Singleton::m_mutex;

// 指令重排序
Singleton* Singleton::getInstance() {
    if (m_instance == nullptr) {
        unique_lock<mutex> lock(m_mutex);
        if (m_instance == nullptr) {
            m_instance = new Singleton();
        }
    }
    return m_instance;
}

int main()
{
    Singleton *s1 = Singleton::getInstance();
    Singleton *s2 = Singleton::getInstance();
    s1->a++;
    s2->a++;

    return 0;
}

静态变量

#include <bits/stdc++.h>

using namespace std;

class Singleton{
private:
    Singleton() = default;
    Singleton(const Singleton& other) = default;
public:
    static Singleton* getInstance();
    static Singleton* m_instance;
    int a = 0;
};

Singleton* Singleton::m_instance = new Singleton();

// 线程安全
Singleton* Singleton::getInstance() {
    return m_instance;
}

int main()
{
    Singleton *s1 = Singleton::getInstance();
    Singleton *s2 = Singleton::getInstance();
    s1->a++;
    s2->a++;

    return 0;
}