C++11的新特性：右值引用

朴灿烈づ我的快乐病毒、 2022-01-05 22:15 234阅读 0赞

先看代码

#include "pch.h"
    #include <iostream>
    #include <string>
    using namespace std;
    
    template <typename elemType>
    class MyArray 
    {
    public:
        MyArray(int capacity);
        MyArray(const MyArray<elemType>& arr);
        ~MyArray();
        elemType& operator[](int index);
        MyArray<elemType> operator=(const MyArray<elemType>& arr);
        void PushBack(elemType& data);
        int GetSize() const { return this->mSize; }
        //void PushBack(T&& data);
    private:
        int mCapacity;
        int mSize;
        elemType *pAddr;
    };
    
    template <typename elemType>
    MyArray<elemType>::MyArray(int capacity)
    {
        this->mCapacity = capacity;
        this->mSize = 0;
        this->pAddr = new elemType[this->mCapacity];
    }
    
    template <typename elemType>
    elemType& MyArray<elemType>::operator[](int index)
    {
        if (index > this->mCapacity || index < 0)
        {
            //异常
        }
        else
        {
            return *(this->pAddr + index);
        }
    }
    
    template<typename elemType>
    void MyArray<elemType>::PushBack(elemType& data)
    {
        if (this->mSize >= this->mCapacity)
        {
            //异常
            return;
        }
        else
        {
            *(this->pAddr + this->mSize) = data;
            this->mSize++;
        }
    }
    
    template<typename elemType>
    MyArray<elemType>::MyArray(const MyArray<elemType>& arr)
    {
        this->mCapacity = arr.mCapacity;
        this->mSize = arr.mSize;
        //申请内存空间
        this->pAddr = new elemType[this->mCapacity];
        //数据拷贝
        for (int ix = 0; ix < this->mSize; ++ix)
        {
            this->pAddr[ix] = arr.pAddr[ix];
        }
    }
    
    template<typename elemType>
    MyArray<elemType>::~MyArray()
    {
        if (this->pAddr != NULL)
        {
            delete[] this->pAddr;
        }
    }
    
    template<typename elemType>
    MyArray<elemType> MyArray<elemType>::operator=(const MyArray<elemType>& arr)
    {
        if (this->pAddr != NULL)
        {
            delete[] this->pAddr;
        }
        this->mCapacity = arr.mCapacity;
        this->mSize = arr.mSize;
        //申请内存空间
        this->pAddr = new elemType[this->mCapacity];
        //数据拷贝
        for (int ix = 0; ix < this->mSize; ++ix)
        {
            this->pAddr[ix] = arr.pAddr[ix];
        }
        return *this;
    }
    
    
    
    void test01()
    {
        MyArray<int> marray(20);
        int a = 10;
        int b = 20;
        int c = 30;
        int d = 40;
        marray.PushBack(a);
        marray.PushBack(b);
        marray.PushBack(c);
        marray.PushBack(d);
        marray.PushBack(100);
        marray.PushBack(200);
    
        for (int ix = 0; ix < marray.GetSize(); ++ix)
        {
            cout << marray[ix] << " ";
        }
    }
    
    int main()
    {
        test01();
        return 0;
    }

代码模拟了STL中的array容器，编译代码，报错  
![1327401-20190619112848543-1290791738.png][]  
报错的代码为

marray.PushBack(100);
        marray.PushBack(200);

PushBack()的实现如下

template<typename elemType>
    void MyArray<elemType>::PushBack(elemType& data)
    {
        if (this->mSize >= this->mCapacity)
        {
            //异常
            return;
        }
        else
        {
            *(this->pAddr + this->mSize) = data;
            this->mSize++;
        }
    }

其参数为引用，不能对右值取引用，也就是说

int i = &42;

这行代码是错误的。

//不能对右值取引用
        //左值 可以在多行使用
        //右值 即临时变量，只能在当前行使用
        marray.PushBack(100);
        marray.PushBack(200);

解决办法：重载PushBack()函数

template<typename elemType>
    void MyArray<elemType>::PushBack(elemType && data)
    {
        if (this->mSize >= this->mCapacity)
        {
            //异常
            return;
        }
        else
        {
            *(this->pAddr + this->mSize) = data;
            this->mSize++;
        }
    }

**另：**  
在VS2017开发环境中，将PushBack()的函数实现如下

void PushBack(const elemType& data);    //类内声明
    
    template<typename elemType>  //类外实现
    void MyArray<elemType>::PushBack(const elemType& data)
    {
        if (this->mSize >= this->mCapacity)
        {
            //异常
            return;
        }
        else
        {
            *(this->pAddr + this->mSize) = data;
            this->mSize++;
        }
    }

这样在使用PushBack()时，编译不会报错

marray.PushBack(100);
        marray.PushBack(200);

但在Linux下，gcc版本为4.4.6，即便是写为

void PushBack(const elemType& data);    //类内声明

编译器仍旧会报错。  
![1327401-20190619150800081-2059940782.png][]

转载于:https://www.cnblogs.com/Manual-Linux/p/11050387.html

[1327401-20190619112848543-1290791738.png]: /images/20211228/5314631b7cc144c6a30719367082da8b.png
[1327401-20190619150800081-2059940782.png]: /images/20211228/0722c5424f324f72b7344a3f09de6cca.png