c++第四次实验：运算符重载

#include 
using namespace std;

class String{
	protected:
		char *s;
	public:
		String(){s=NULL;}//定义默认构造函数 
		String(char *str);//声明构造函数 
		String(const String &stri);//拷贝构造函数 
		~String();//析构函数 
		String operator=(String &str);//成员函数重载= 
		String operator+=(String &str);//成员函数重载+= 
		friend String operator+(String &str1,String &str2);//友元函数重载+ 
		friend bool operator>(String &str1,String &str2);//友元函数重载>
		friend bool operator>=(String &str1,String &str2);//友元函数重载>=  
		friend bool operator<(String &str1,String &str2);//友元函数重载<
		friend bool operator<=(String &str1,String &str2);//友元函数重载<=
		friend bool operator==(String &str1,String &str2);//友元函数重载==
		friend bool operator!=(String &str1,String &str2);//友元函数重载!=
		void display();
};

char* Stract(char *str1,char*str2)//自定义字符串连接函数 
{
	char *str;
	int i,j;
	int len1 = strlen(str1);
	int len2 = strlen(str2);
	int len = len1+len2;
	str = new char(len+1);
	strcpy(str,str1);
	for(i=0,j=len1;i(String &str1,String &str2)
{
	if(strcmp(str1.s,str2.s)>0) return true;
	else{
		return false;
	}
}

bool operator>=(String &str1,String &str2)
{
	if(strcmp(str1.s,str2.s)>=0) return true;
	else{
		return false;
	}
}

bool operator<(String &str1,String &str2)
{
	if(strcmp(str1.s,str2.s)<0) return true;
	else{
		return false;
	}
}

bool operator<=(String &str1,String &str2)
{
	if(strcmp(str1.s,str2.s)<=0) return true;
	else{
		return false;
	}
}

bool operator==(String &str1,String &str2)
{
	if(strcmp(str1.s,str2.s)==0) return true;
	else{
		return false;
	}
}

bool operator!=(String &str1,String &str2)
{
	if(strcmp(str1.s,str2.s)!=0) return true;
	else{
		return false;
	}
}

void String::display()
{
	cout<