移動語義 && 函數調用過程中的 lvalue，c移動語義

移動語義 && 函數調用過程中的 lvalue，c移動語義

當以一個函數內的臨時變量對象作為另一個函數的形參的時候，原函數內的臨時對象即 rvalue，就會成為此函數內的 lvalue。

這樣會重新導致效率低下，因為造成了大量復制操作。

<utility>頭文件提供了 std：move（）函數。此函數返回作為 rvalue 傳遞給的任何實參。

觀察下面程序的輸出：

class CText
{
private:
    char *pText;

public:
    void showIt()const
    {
        cout << pText << endl;
    }

    CText(const char* pStr = "No text")
    {
        cout << "CText constructor called" << endl;
        size_t len{ strlen(pStr) + 1 };
        pText = new char[len];
        strcpy_s(pText, len, pStr);
    }

    CText(const CText & txt)
    {
        cout << "CText copy constructor called" << endl;
        size_t len{ strlen(txt.pText) + 1 };
        pText = new char[len];
        strcpy_s(pText, len, txt.pText);
    }

    CText(CText && txt)
    {
        cout << "CText move constructor called" << endl;
        pText = txt.pText;
        txt.pText = nullptr;
    }

    ~CText()
    {
        cout << "CText destructor called" << endl;
        delete[]pText;
    }

    CText & operator=(const CText & txt)
    {
        cout << "CText assignment operator function called" << endl;
        if (this != &txt)
        {
            delete[]pText;
            size_t length{ strlen(txt.pText) + 1 };
            pText = new char[length];
            strcpy_s(pText, length, txt.pText);
        }
        return *this;
    }

    CText & operator=(CText && txt)
    {
        cout << "CText move assignment operator function called" << endl;
        delete[]pText;
        pText = txt.pText;
        txt.pText = nullptr;
        return *this;
    }

    CText operator+(const CText & txt)const
    {
        cout << "CText add operator function called" << endl;
        size_t length{ strlen(pText) + strlen(txt.pText) + 1 };
        CText aText;
        aText.pText = new char[length];
        strcpy_s(aText.pText, length, pText);
        strcat_s(aText.pText, length, txt.pText);
        return aText;
    }
};

CText 實現了移動語義的復制構造和賦值運算符函數，並且CText的對象作為CMessage類的成員。

class CMessage
{
private:
    CText  m_Text;

public:
    void showIt()const
    {
        m_Text.showIt();
    }

    CMessage operator+(const CMessage & aMess) const
    {
        cout << "CMessage add operator function called" << endl;
        CMessage message;
        message.m_Text = m_Text + aMess.m_Text;
        return message;

    }

    CMessage & operator=(const CMessage & aMess)
    {
        cout << "CMessage assignment operator function called" << endl;
        if (this != &aMess)
        {
            m_Text = aMess.m_Text;
        }
        return *this;
    }

    CMessage & operator=(CMessage && aMess)
    {
        cout << "CMessage move assignment operator function called" << endl;
        m_Text = aMess.m_Text;
        return *this;
    }

    CMessage(const char * str = "Default message")//:m_Text{ str }//m_Text { CText(str) }
    {
        cout << "CMessage constructor called----" << endl;
        m_Text = CText(str);
    }

    CMessage(const CMessage & amess)
    {
        cout << "cmessage copy constructor called" << endl;
        m_Text = amess.m_Text;
    }

    CMessage(const CMessage && amess)
    {
        cout << "cmessage move constructor called" << endl;
        m_Text = amess.m_Text;
    }
};

int main()
{
    CMessage motto1{"The devi1 takes care of his own.\n"};

    cout << "----------------------------------------" << endl;

    CMessage motto2{"if yuo sup with the devil use a long spoon.\n"};

    cout << "----------------------------------------" << endl;

    CMessage motto3{motto1+motto2};

    cout << "----------------------------------------" << endl;

    motto3.showIt();
}

 注意：CMessage 的構造函數，用初始化列表和在構造函數中初始化的差別。

當用初始化列表的輸出如下：