模板函数例子以及友元函数滥用的问题

使用模板函数实现一个复数类，熟悉模板函数的使用：

（1）在类的内部实现

#include 
using namespace std;

template 
class Complex
{
	friend ostream & operator<< (ostream &out, Complex c)
	{
		out << c.a << "+" << c.b << endl;
		return out;
	}
public:
	Complex(T a, T b)
	{
		this->a = a;
		this->b = b;
	}

	Complex operator+ (Complex &c)
	{
		Complex tem(a+c.a,b+c.b);
		return tem;
	}
private:
	T a;
	T b;

};


int main()
{
	Complex c1(1, 2), c2(3, 4);
	Complex c3 = c1 + c2;

	cout << c3 << endl;

	system("pause");
	return 0;
}

（2）在类的外部实现（在一个.cpp文件）

#include 
using namespace std;

template 
class Complex
{
	friend ostream & operator<<  (ostream &out, Complex &c);//注意此处位置,若无则报错
public:
	Complex(T a, T b);

	Complex operator+ (Complex &c);
	Complex operator- (Complex &c);

private:
	T a;
	T b;
};

template 
Complex::Complex(T a, T b)
{
	this->a = a;
	this->b = b;
}

template 
Complex Complex::operator+ (Complex &c)
{
	Complex tem(a + c.a, b + c.b);
	return tem;
}

template 
Complex Complex::operator- (Complex &c)
{
	Complex tem(a-c.a,b-c.b);
	return tem;
}

template 
ostream & operator<< (ostream &out, Complex &c)
{
	out << c.a << " + " << c.b;
	return out;
}

int main()
{
	Complex c1(1,2);
	Complex c2(3, 4);

	Complex c3 = c1 + c2;
	Complex c4 = c2 - c1;

	cout << c3 << endl;
	cout << c4 << endl;

	
	system("pause");


	return 0;
}

需特别注意重载<<操作符时用到了友元函数，此时必须在operator<<后加上，否则无法编译通过

不可以滥用友元函数（不是使用友元函数重载<< >>的情况），否则会产生错误，此时必须在类的前面添加类的前置声明以及函数的前置声明：

template 
class Complex;
template 
Complex mySub(Complex&c1, Complex&c2);

同时在类的内部的声明必须写成：

friend Complex mySub(Complex&c1, Complex&c2);

友元函数的实现写成：

template 
Complex mySub(Complex&c1, Complex&c2)
{
	Complex tem(c1.a - c2.a,c1.b - c2.b);
	return tem;
}

友元函数的调用写成：

Complex c5 = mySub(c2, c1);

（3）所有函数都写在类的外部（.h和.cpp分开）

.h文件

#pragma once
#include 
using namespace std;

template 
class Complex
{
	friend ostream & operator<<  (ostream &out, Complex &c);//注意此处位置,若无则报错
public:
	Complex(T a, T b);

	Complex operator+ (Complex &c);
	Complex operator- (Complex &c);

private:
	T a;
	T b;
};

.cpp文件：

#include "Complex.h"

template 
Complex::Complex(T a, T b)
{
	this->a = a;
	this->b = b;
}

template 
Complex Complex::operator+ (Complex &c)
{
	Complex tem(a + c.a, b + c.b);
	return tem;
}

template 
Complex Complex::operator- (Complex &c)
{
	Complex tem(a - c.a, b - c.b);
	return tem;
}

template 
ostream & operator<< (ostream &out, Complex &c)
{
	out << c.a << " + " << c.b;
	return out;
}

main文件：

#include "Complex.cpp"//注意此处引入的是.cpp文件

int main()
{
	Complex c1(1,2);
	Complex c2(3, 4);
	Complex c3 = c1 + c2;

	cout << c3 << endl;
	system("pause");

	return 0;
}

注意引入文件时引入的是.cpp文件，否则会报找不到函数

（4）模板函数中的静态变量

不同类型各自使用自己的静态变量