第8周項目4-String類的構造

/*。
*Copyright(c)2014,煙臺大學計算機學院
*All right reserved,
*文件名：test.cpp
*作者：畢玉堂
*完成日期：2015年5月7日
*版本號：v1.0
*
問題描述：
*輸入描述：
*程序輸出：
*/
#include <iostream>
#include <Cstring>
using namespace std;
class String
{
public:
    String( );                         //默認構造函數
    String(const char *s);
    String(String &str);               //構造函數
    ~String();
    void display( );
    friend String operator + (String &s1,String &s2 );
    friend String operator - (String &s1,String &s2 );
private:
    char *p;
    int len;                                       //字符型指針，用於指向字符串
};
String::String()
{
    len = 0;
    p = NULL;
}
String::String(const char *s)
{
    len = strlen(s);
    p = new char[len+1];
    strcpy(p,s);
}

String::String(String &str)
{
    len = str.len;
    if(p!=NULL) delete []p;   //當發生在賦值等情形時，原對象可能已經存在，需釋放原有空間
    p = new char[len+1];
    strcpy(p,str.p);
}

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

void String::display( )     //輸出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];
    strcpy(s.p,s1.p);
    strcat(s.p,s2.p);
    return s;
}

String operator - (String &s1, String &s2 )
{
    String s;
    //c1爲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爲去除前導空格的字符串
    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';

    //將這兩部分接起來
    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;
}

運行結果：