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C++單例模式的幾種實現(xiàn)方法詳解_C 語言

作者：戀戀風(fēng)辰 ? 更新時間： 2022-05-23 編程語言

局部靜態(tài)變量方式

//通過靜態(tài)成員變量實現(xiàn)單例
//懶漢式
class Single2
{
private:
    Single2()
    {
    }
    Single2(const Single2 &) = delete;
    Single2 &operator=(const Single2 &) = delete;
public:
    static Single2 &GetInst()
    {
        static Single2 single;
        return single;
    }
};

上述代碼通過局部靜態(tài)成員single實現(xiàn)單例類，原理就是函數(shù)的局部靜態(tài)變量生命周期隨著進(jìn)程結(jié)束而結(jié)束。上述代碼通過懶漢式的方式實現(xiàn)。

調(diào)用如下

void test_single2()
{
    //多線程情況下可能存在問題
    cout << "s1 addr is " << &Single2::GetInst() << endl;
    cout << "s2 addr is " << &Single2::GetInst() << endl;
}

程序輸出如下

sp1 ?is ?0x1304b10
sp2 ?is ?0x1304b10

確實生成了唯一實例，上述單例模式存在隱患，對于多線程方式生成的實例可能時多個。

靜態(tài)成員變量指針方式

可以定義一個類的靜態(tài)成員變量，用來控制實現(xiàn)單例

//餓漢式
class Single2Hungry
{
private:
    Single2Hungry()
    {
    }
    Single2Hungry(const Single2Hungry &) = delete;
    Single2Hungry &operator=(const Single2Hungry &) = delete;
public:
    static Single2Hungry *GetInst()
    {
        if (single == nullptr)
        {
            single = new Single2Hungry();
        }
        return single;
    }
private:
    static Single2Hungry *single;
};

這么做的一個好處是我們可以通過餓漢式的方式避免線程安全問題

//餓漢式初始化
Single2Hungry *Single2Hungry::single = Single2Hungry::GetInst();
void thread_func_s2(int i)
{
    cout << "this is thread " << i << endl;
    cout << "inst is " << Single2Hungry::GetInst() << endl;
}
void test_single2hungry()
{
    cout << "s1 addr is " << Single2Hungry::GetInst() << endl;
    cout << "s2 addr is " << Single2Hungry::GetInst() << endl;
    for (int i = 0; i < 3; i++)
    {
        thread tid(thread_func_s2, i);
        tid.join();
    }
}
int main(){
    test_single2hungry()
}

程序輸出如下

s1 addr is 0x1e4b00
s2 addr is 0x1e4b00
this is thread 0
inst is 0x1e4b00
this is thread 1
inst is 0x1e4b00
this is thread 2
inst is 0x1e4b00

可見無論單線程還是多線程模式下，通過靜態(tài)成員變量的指針實現(xiàn)的單例類都是唯一的。餓漢式是在程序啟動時就進(jìn)行單例的初始化，這種方式也可以通過懶漢式調(diào)用，無論餓漢式還是懶漢式都存在一個問題，就是什么時候釋放內(nèi)存？多線程情況下，釋放內(nèi)存就很難了，還有二次釋放內(nèi)存的風(fēng)險。

我們定義一個單例類并用懶漢式方式調(diào)用

//懶漢式指針
//即使創(chuàng)建指針類型也存在問題
class SinglePointer
{
private:
    SinglePointer()
    {
    }
    SinglePointer(const SinglePointer &) = delete;
    SinglePointer &operator=(const SinglePointer &) = delete;
public:
    static SinglePointer *GetInst()
    {
        if (single != nullptr)
        {
            return single;
        }
        s_mutex.lock();
        if (single != nullptr)
        {
            s_mutex.unlock();
            return single;
        }
        single = new SinglePointer();
        s_mutex.unlock();
        return single;
    }
private:
    static SinglePointer *single;
    static mutex s_mutex;
};

在cpp文件里初始化靜態(tài)成員,并定義一個測試函數(shù)

//懶漢式
//在類的cpp文件定義static變量
SinglePointer *SinglePointer::single = nullptr;
std::mutex SinglePointer::s_mutex;
void thread_func_lazy(int i)
{
    cout << "this is lazy thread " << i << endl;
    cout << "inst is " << SinglePointer::GetInst() << endl;
}
void test_singlelazy()
{
    for (int i = 0; i < 3; i++)
    {
        thread tid(thread_func_lazy, i);
        tid.join();
    }
    //何時釋放new的對象？造成內(nèi)存泄漏
}
int main(){
    test_singlelazy();
}

函數(shù)輸出如下

this is lazy thread 0
inst is 0xbc1700
this is lazy thread 1
inst is 0xbc1700
this is lazy thread 2
inst is 0xbc1700

此時生成的單例對象的內(nèi)存空間還沒回收，這是個問題，另外如果多線程情況下多次delete也會造成崩潰。

智能指針方式

可以利用智能指針自動回收內(nèi)存的機(jī)制設(shè)計單例類

//利用智能指針解決釋放問題
class SingleAuto
{
private:
    SingleAuto()
    {
    }
    SingleAuto(const SingleAuto &) = delete;
    SingleAuto &operator=(const SingleAuto &) = delete;
public:
    ~SingleAuto()
    {
        cout << "single auto delete success " << endl;
    }
    static std::shared_ptr GetInst()
    {
        if (single != nullptr)
        {
            return single;
        }
        s_mutex.lock();
        if (single != nullptr)
        {
            s_mutex.unlock();
            return single;
        }
        single = std::shared_ptr(new SingleAuto);
        s_mutex.unlock();
        return single;
    }
private:
    static std::shared_ptr single;
    static mutex s_mutex;
};

SingleAuto的GetInst返回std::shared_ptr類型的變量single。因為single是靜態(tài)成員變量，所以會在進(jìn)程結(jié)束時被回收。智能指針被回收時會調(diào)用內(nèi)置指針類型的析構(gòu)函數(shù)，從而完成內(nèi)存的回收。

在主函數(shù)調(diào)用如下測試函數(shù)

// 智能指針方式
std::shared_ptr SingleAuto::single = nullptr;
mutex SingleAuto::s_mutex;
void test_singleauto()
{
    auto sp1 = SingleAuto::GetInst();
    auto sp2 = SingleAuto::GetInst();
    cout << "sp1  is  " << sp1 << endl;
    cout << "sp2  is  " << sp2 << endl;
    //此時存在隱患，可以手動刪除裸指針，造成崩潰
    // delete sp1.get();
}
int main(){
    test_singleauto();
}

程序輸出如下

sp1 ?is ?0x1174f30
sp2 ?is ?0x1174f30

智能指針方式不存在內(nèi)存泄漏，但是有一個隱患就是單例類的析構(gòu)函數(shù)時public的，如果被人手動調(diào)用會存在崩潰問題，比如將上邊test_singleauto中的注釋打開，程序會崩潰。

輔助類智能指針單例模式

智能指針在構(gòu)造的時候可以指定刪除器，所以可以傳遞一個輔助類或者輔助函數(shù)幫助智能指針回收內(nèi)存時調(diào)用我們指定的析構(gòu)函數(shù)。

// safe deletor
//防止外界delete
//聲明輔助類
//該類定義仿函數(shù)調(diào)用SingleAutoSafe析構(gòu)函數(shù)
//不可以提前聲明SafeDeletor，編譯時會提示incomplete type
// class SafeDeletor;
//所以要提前定義輔助類
class SingleAutoSafe;
class SafeDeletor
{
public:
    void operator()(SingleAutoSafe *sf)
    {
        cout << "this is safe deleter operator()" << endl;
        delete sf;
    }
};
class SingleAutoSafe
{
private:
    SingleAutoSafe() {}
    ~SingleAutoSafe()
    {
        cout << "this is single auto safe deletor" << endl;
    }
    SingleAutoSafe(const SingleAutoSafe &) = delete;
    SingleAutoSafe &operator=(const SingleAutoSafe &) = delete;
    //定義友元類，通過友元類調(diào)用該類析構(gòu)函數(shù)
    friend class SafeDeletor;
public:
    static std::shared_ptr GetInst()
    {
        if (single != nullptr)
        {
            return single;
        }
        s_mutex.lock();
        if (single != nullptr)
        {
            s_mutex.unlock();
            return single;
        }
        //額外指定刪除器
        single = std::shared_ptr(new SingleAutoSafe, SafeDeletor());
        //也可以指定刪除函數(shù)
        // single = std::shared_ptr(new SingleAutoSafe, SafeDelFunc);
        s_mutex.unlock();
        return single;
    }
private:
    static std::shared_ptr single;
    static mutex s_mutex;
};

SafeDeletor要寫在SingleAutoSafe上邊，并且SafeDeletor要聲明為SingleAutoSafe類的友元類，這樣就可以訪問SingleAutoSafe的析構(gòu)函數(shù)了。

我們在構(gòu)造single時制定了SafeDeletor(),single在回收時，會調(diào)用SingleAutoSafe的仿函數(shù)，從而完成內(nèi)存的銷毀。

并且SingleAutoSafe的析構(gòu)函數(shù)為私有的無法被外界手動調(diào)用了。

//智能指針初始化為nullptr
std::shared_ptr SingleAutoSafe::single = nullptr;
mutex SingleAutoSafe::s_mutex;
void test_singleautosafe()
{
    auto sp1 = SingleAutoSafe::GetInst();
    auto sp2 = SingleAutoSafe::GetInst();
    cout << "sp1  is  " << sp1 << endl;
    cout << "sp2  is  " << sp2 << endl;
    //此時無法訪問析構(gòu)函數(shù)，非常安全
    // delete sp1.get();
}
int main(){
    test_singleautosafe();
}

程序輸出如下

sp1  is  0x1264f30
sp2  is  0x1264f30

通過輔助類調(diào)用單例類的析構(gòu)函數(shù)保證了內(nèi)存釋放的安全性和唯一性。這種方式時生產(chǎn)中常用的。如果將test_singleautosafe函數(shù)的注釋打開，手動delete sp1.get()編譯階段就會報錯，達(dá)到了代碼安全的目的。因為析構(gòu)被設(shè)置為私有函數(shù)了。

通用的單例模板類

我們可以通過聲明單例的模板類，然后繼承這個單例模板類的所有類就是單例類了。達(dá)到泛型編程提高效率的目的。

template 
class Single_T
{
protected:
    Single_T() = default;
    Single_T(const Single_T &st) = delete;
    Single_T &operator=(const Single_T &st) = delete;
    ~Single_T()
    {
        cout << "this is auto safe template destruct" << endl;
    }
public:
    static std::shared_ptr GetInst()
    {
        if (single != nullptr)
        {
            return single;
        }
        s_mutex.lock();
        if (single != nullptr)
        {
            s_mutex.unlock();
            return single;
        }
        //額外指定刪除器
        single = std::shared_ptr(new T, SafeDeletor_T());
        //也可以指定刪除函數(shù)
        // single = std::shared_ptr(new SingleAutoSafe, SafeDelFunc);
        s_mutex.unlock();
        return single;
    }
private:
    static std::shared_ptr single;
    static mutex s_mutex;
};
//模板類的static成員要放在h文件里初始化
template 
std::shared_ptr Single_T::single = nullptr;
template 
mutex Single_T::s_mutex;

我們定義一個網(wǎng)絡(luò)的單例類，繼承上述模板類即可，并將構(gòu)造和析構(gòu)設(shè)置為私有，同時設(shè)置友元保證自己的析構(gòu)和構(gòu)造可以被友元類調(diào)用.

//通過繼承方式實現(xiàn)網(wǎng)絡(luò)模塊單例
class SingleNet : public Single_T
{
private:
    SingleNet() = default;
    SingleNet(const SingleNet &) = delete;
    SingleNet &operator=(const SingleNet &) = delete;
    ~SingleNet() = default;
    friend class SafeDeletor_T;
    friend class Single_T;
};

在主函數(shù)中調(diào)用如下

void test_singlenet()
{
    auto sp1 = SingleNet::GetInst();
    auto sp2 = SingleNet::GetInst();
    cout << "sp1  is  " << sp1 << endl;
    cout << "sp2  is  " << sp2 << endl;
}

程序輸出如下

sp1 ?is ?0x1164f30
sp2 ?is ?0x1164f30

總結(jié)

原文鏈接：https://blog.csdn.net/secondtonone1/article/details/123616175

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