String 类的构造

/* Copyright (c) 2016* All rights reserved 烟台大学计算机与控制工程学院
* 文件名称：3.cpp
* 作者：刘丽
* 完成日期：2016年 5 月 24日
* 版本号： v1.0
*【项目-String类的构造】
写一个能处理字符串的类，其数据成员如下所示：
*/

#include <iostream>
#include <Cstring>
using namespace std;
class String
{
public:
String( );
String(const char *s);
String(const String &str);

~String();
void display( );
friend String operator + (String &s1,String &s2 );
friend String operator - (String &s1,String &s2 );
private:
char *p;
int len;                                       //字符型指针，用于指向字符串
};
String::String()
{      len = 0;
p = NULL;
}
String::String(const char *s)
{
len = strlen(s);
p = new char[len+1];
strcpy(p,s);
}

String::String(const String &str)
{
len = str.len;
if(p!=NULL) delete []p;   //当发生在赋值等情形时，原对象可能已经存在，需释放原有空间
p = new char[len+1];
strcpy(p,str.p);
}

String::~String()
{
if(!p) delete []p;
}

void String::display( )     //输出p所指向的字符串
{
cout<<p<<endl;
}

String operator + (String &s1, String &s2 )
{
String s;
s.len = s1.len+s2.len;
s.p = new char[s.len+1];   //原误写char(s.len+1)
strcpy(s.p,s1.p);
strcat(s.p,s2.p);
return s;
}

String operator - (String &s1, String &s2 )
{
String s;
//c1为截去尾部空格的字符串
char *c1=new char[s1.len+1];
strcpy(c1,s1.p);
int i=s1.len-1;
while(i>=0&&c1[i]==' ') --i;
c1[i+1]='\0';

//c2为去除前导空格的字符串
char *c2=new char[s2.len+1];
strcpy(c2,s2.p);
i=0;
while(i<s2.len&&c2[i]==' ') ++i;
int j=0;
while(i<s2.len&&c2[i]!='\0')
{
c2[j]=c2[i];
++i;
++j;
}
c2[j]='\0';

//将这两部分接起来
s.len = strlen(c1)+strlen(c2);
s.p = new char[s.len+1];
strcpy(s.p,c1);
strcat(s.p,c2);
delete c1;
delete c2;
return s;
}

int main( )
{
String string1(" Hello  "), string2(" World ");
string1.display();
string2.display();
String string3;
string3 = string1 + string2;
string3.display();
string3 = string1 - string2;
string3.display();
return 0;
}