C++11的新特性：右值引用

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

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。