变参模板改进单例模式

单例模式保证一个类仅有一个实例，并提供一个访问它的全局访问点

泛型单例模式需要变参构造函数，构造函数的参数个数需要支持变化

下面是不用变参模板，支持0～6个参数的单例模式实现

#include <iostream>

// 泛型单例模式需要变参构造函数，构造函数的参数个数需要支持变化
// 支持0～6个参数的单例模式实现

template <typename T>
class Singleton
{
public:
    // 支持0个参数的构造函数
    static T *Instance()
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T();
        return m_pInstance;
    }

    // 支持1个参数的构造函数
    template <typename T0>
    static T *Instance(T0 arg0)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(arg0);
        return m_pInstance;
    }

    // 支持2个参数的构造函数
    template <typename T0, typename T1>
    static T *Instance(T0 arg0, T1 arg1)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(arg0, arg1);
        return m_pInstance;
    }

    // 支持3个参数的构造函数
    template <typename T0, typename T1, typename T2>
    static T *Instance(T0 arg0, T1 arg1, T2 arg2)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(arg0, arg1, arg2);
        return m_pInstance;
    }

    // 支持4个参数的构造函数
    template <typename T0, typename T1, typename T2, typename T3>
    static T *Instance(T0 arg0, T1 arg1, T2 arg2, T3 arg3)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(arg0, arg1, arg2, arg3);
        return m_pInstance;
    }

    // 支持5个参数的构造函数
    template <typename T0, typename T1, typename T2, typename T3, typename T4>
    static T *Instance(T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(arg0, arg1, arg2, arg3, arg4);
        return m_pInstance;
    }

    // 支持6个参数的构造函数
    template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5>
    static T *Instance(T0 arg0, T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(arg0, arg1, arg2, arg3, arg4, arg5);
        return m_pInstance;
    }

    // 获取单例
    static T GetInstance()
    {
        if (m_pInstance == nullptr)
        {
            std::cout << "the instance is not init, please initialize the instance first" << std::endl;
            return nullptr;
        }
        return m_pInstance;
    }

    // 释放单例
    static void DestroyInstance()
    {
        delete m_pInstance;
        m_pInstance = nullptr;
    }

private:
    // 不允许赋值和复制
    Singleton(void);
    virtual ~Singleton(void);
    Singleton(const Singleton &);
    Singleton &operator=(const Singleton &);

    static T *m_pInstance;
};

template <class T>
T *Singleton<T>::m_pInstance = nullptr;

// test data
struct A
{
    A() {}
};

struct B
{
    B(int x) {}
};

struct C
{
    C(int x, double y) {}
};

int main(void)
{
    Singleton<A>::Instance();
    Singleton<B>::Instance(1);
    Singleton<C>::Instance(1, 3.14);

    Singleton<A>::DestroyInstance();
    Singleton<B>::DestroyInstance();
    Singleton<C>::DestroyInstance();

    return 0;
}

通过变参模板改进单例模式

#include <iostream>
#include <string>

template <typename T>
class Singleton
{
public:
    template <typename... Args>
    static T *Instance(Args &&...args)
    {
        if (m_pInstance == nullptr)
            m_pInstance = new T(std::forward<Args>(args)...);
        return m_pInstance;
    }

    // 获取单例
    static T *GetInstance()
    {
        if (m_pInstance == nullptr)
        {
            std::cout << "the instance is not init, please initialize the instance first" << std::endl;
            return nullptr;
        }
        return m_pInstance;
    }

    static void DestoryInstance()
    {
        delete m_pInstance;
        m_pInstance = nullptr;
    }

private:
    Singleton(void);
    virtual ~Singleton(void);
    Singleton(const Singleton &);
    Singleton &operator=(const Singleton &);
    static T *m_pInstance;
};

template <typename T>
T *Singleton<T>::m_pInstance = nullptr;

// test data
struct A
{
    A(const std::string &)
    {
        std::cout << "lvalue" << std::endl;
    }
    A(std::string &&x)
    {
        std::cout << "rvalue" << std::endl;
    }
};

struct B
{
    B(const std::string &)
    {
        std::cout << "lvalue" << std::endl;
    }
    B(std::string &&x)
    {
        std::cout << "rvalue" << std::endl;
    }
};

struct C
{
    C(int x, double y) {}
    void Fun() { std::cout << "test" << std::endl; }
};

int main(void)
{
    std::string str = "bb";

    // 创建A类型的单例
    Singleton<A>::Instance(str);

    // 创建B类型的单例
    Singleton<B>::Instance(std::move(str));

    // 创建C类型的单例
    Singleton<C>::Instance(1, 3.14);

    // 获取单例并调用单例对象的方法
    Singleton<C>::GetInstance()->Fun();

    // 释放单例
    Singleton<A>::DestoryInstance();
    Singleton<B>::DestoryInstance();
    Singleton<C>::DestoryInstance();
}

编译运行

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