C++模板类与运算符作为友元函数重载

我自定义了一个模板类并重载了运算符，预定义实现功能为能实现对数组一般操作，类似于vector。

#ifndef ARRAY_H#define ARRAY_H#include <iostream>using namespace std;template<class T>class Array{friend ostream& operator<<(ostream&, const Array &);friend istream& operator>>(istream&, Array &);public:Array(int arraySize = 10){size = (arraySize > 0 ? arraySize : 10);ptr = new T[size];for (int i = 0; i < size; i++)ptr[i] = 0;}Array(const Array  &arrayToCopy){ptr = new int[size];for (int i = 0; i < size; i++){ptr[i] = arrayToCopy[i];}}~Array(){delete[] ptr;}int getSize()const{return size;}const Array& operator=(const Array &right){if (&right != this){if (size != right.size){delete[] ptr;size = right.size;ptr = new T[size];}for (int i = 0; i < size; i++)ptr[i] = right.ptr[i];}return *this;}bool operator==(const Array &right)const{if (size != right.size)return false;for (int i = 0; i < size; i++)if (ptr[i] != right.ptr[i])return false;return true;}bool operator!=(const Array &right)const{return !(*this == right);}T& operator[](int subscript){if (subscript < 0 || subscript >= size){cerr << "\nError: Subscript " << subscript<< "out of range" << endl;exit(1);}return ptr[subscript];}T operator[](int subscript)const{if (subscript < 0 || subscript >= size){cerr << "\nError: Subscript" << subscript<< "out of range" << endl;exit(1);}return ptr[subscript];}private:int size;T *ptr;};#endiftemplate<class T>istream& operator>>(istream &input, Array<T> &a){for (int i = 0; i < a.size; i++)input >> a.ptr[i];return input;}template<class T>ostream& operator<<(ostream &output, const Array<T> &a){int i;for (i = 0; i < a.size; i++){output << setw(12) << a.ptr[i];if ((i + 1) % 4 == 0)ouput << endl;}if (i % 4 != 0)output << endl;return output;}

#include <iostream>#include "Array.h"using namespace std;int main(){Array<int> integers1(7);cout << "Size of Array integers1 is "<< integers1.getSize()<< "\nArray after initialization:\n"<< integers1;}

出错错误原因为

1>main.obj : error LNK2019: 无法解析的外部符号 "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Array<int> const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Array@H@@@Z)，该符号在函数 _main 中被引用

我猜测的原因是cin为ostream对象实例，但是没对我自定义的类型进行<<重载。不知道我的猜测对不对，希望有经验的人给我点文档与链接。

问题已经得到解决，

模板类中的友元函数需要进行特别的处理。原因请参考我的博文。

http://blog.csdn.net/u010003835/article/details/47312955

设置友元后

头文件

#ifndef ARRAY_H#define ARRAY_H#include <iostream>#include <iomanip>using namespace std;//模板类前置声明template<class T>class Array;//Array模板类的友元声明template<class T>istream& operator>>(istream& input, Array<T>& a);template<class T>ostream& operator<<(ostream& output, const Array<T>& a);template<class T>class Array{friend ostream& operator<< <> (ostream&, const Array<T>&);friend istream& operator>> <> (istream&, Array<T>&);public:Array(int arraySize = 10){size = (arraySize > 0 ? arraySize : 10);ptr = new T[size];for (int i = 0; i < size; i++)ptr[i] = 0;}Array(const Array  &arrayToCopy){ptr = new int[size];for (int i = 0; i < size; i++){ptr[i] = arrayToCopy[i];}}~Array(){delete[] ptr;}int getSize()const{return size;}const Array& operator=(const Array &right){if (&right != this){if (size != right.size){delete[] ptr;size = right.size;ptr = new T[size];}for (int i = 0; i < size; i++)ptr[i] = right.ptr[i];}return *this;}bool operator==(const Array &right)const{if (size != right.size)return false;for (int i = 0; i < size; i++)if (ptr[i] != right.ptr[i])return false;return true;}bool operator!=(const Array &right)const{return !(*this == right);}T& operator[](int subscript){if (subscript < 0 || subscript >= size){cerr << "\nError: Subscript " << subscript<< "out of range" << endl;exit(1);}return ptr[subscript];}T operator[](int subscript)const{if (subscript < 0 || subscript >= size){cerr << "\nError: Subscript" << subscript<< "out of range" << endl;exit(1);}return ptr[subscript];}private:int size;T *ptr;};#endiftemplate<class T>istream& operator>>(istream &input, Array<T> &a){for (int i = 0; i < a.size; i++)input >> a.ptr[i];return input;}template<class T>ostream& operator<<(ostream &output, const Array<T> &a){int i;for (i = 0; i < a.size; i++){output  << setw(12) << a.ptr[i];if ((i + 1) % 4 == 0)output << endl;}if (i % 4 != 0)output << endl;return output;}

正确的写法就是代码段中的写法。<>也必不可少，其实<>有两

重意思：

一是，表明此友元函数是函数模板；

二是，此模板使用的模板类型参数为当前

模板类的类型参数class。

主函数

<pre name="code" class="cpp">#include "Array.h"using namespace std;int main(){Array<int> integers1(7);Array<int> integers2;Array<char> char_string(100);cout << "Size of Array integers1 is "<< integers1.getSize()<< "\nArray after initialization:\n" << integers1 << endl;cout << "Size of Array char_string is "<< char_string.getSize()<< "\nArray after initialization:\n" << endl;if (integers1 != integers2)cout << "integers1 and integers2 are not equal" << endl;cout << "\ninteger2[5] is " << integers2[5] << endl;}

运行截图

第二种方法

#ifndef ARRAY_H#define ARRAY_H#include <iostream>#include <iomanip>using namespace std;//模板类前置声明template<class T>class Array;//Array模板类的友元声明template<class S>istream& operator>>(istream& input, Array<S>& a);template<class S>ostream& operator<<(ostream& output, const Array<S>& a);template<class T>class Array{template<class S>friend ostream& operator<<(ostream&, const Array<S>&);template<class S>friend istream& operator>>(istream&, Array<S>&);public:Array(int arraySize = 10){size = (arraySize > 0 ? arraySize : 10);ptr = new T[size];for (int i = 0; i < size; i++)ptr[i] = 0;}Array(const Array  &arrayToCopy){ptr = new int[size];for (int i = 0; i < size; i++){ptr[i] = arrayToCopy[i];}}~Array(){delete[] ptr;}int getSize()const{return size;}const Array& operator=(const Array &right){if (&right != this){if (size != right.size){delete[] ptr;size = right.size;ptr = new T[size];}for (int i = 0; i < size; i++)ptr[i] = right.ptr[i];}return *this;}bool operator==(const Array &right)const{if (size != right.size)return false;for (int i = 0; i < size; i++)if (ptr[i] != right.ptr[i])return false;return true;}bool operator!=(const Array &right)const{return !(*this == right);}T& operator[](int subscript){if (subscript < 0 || subscript >= size){cerr << "\nError: Subscript " << subscript<< "out of range" << endl;exit(1);}return ptr[subscript];}T operator[](int subscript)const{if (subscript < 0 || subscript >= size){cerr << "\nError: Subscript" << subscript<< "out of range" << endl;exit(1);}return ptr[subscript];}private:int size;T *ptr;};#endiftemplate<class T>istream& operator>>(istream &input, Array<T> &a){for (int i = 0; i < a.size; i++)input >> a.ptr[i];return input;}template<class T>ostream& operator<<(ostream &output, const Array<T> &a){int i;for (i = 0; i < a.size; i++){output << setw(12) << a.ptr[i];if ((i + 1) % 4 == 0)output << endl;}if (i % 4 != 0)output << endl;return output;}