程序代码：

#include <iostream.h>
#include <string.h>

class yang
{
public:
    yang();
    yang(const char *const);
    yang(const yang&);
    ~yang();
    char & operator[](unsigned short offset);
    char operator[](unsigned short offset)const;
    yang operator+(const yang&);
    void operator+=(const yang&);
    yang& operator=(const yang&);

    unsigned short getlen()const{return itslen;}
    const char* getyang(){return itsyang;}
private:
    yang(unsigned short);
    char *itsyang;
    unsigned short itslen;
};
yang::yang()
{
    itsyang= new char[1];
    itsyang[0]='\0';
    itslen=0;
}
yang::yang(unsigned short len)
{
    itsyang=new char[len+1];
    for(unsigned short i=0; i<len; i++)
        itsyang[i]='\0';
    itslen=len;
}
yang::yang(const char * const cyang)
{
    itslen=strlen(cyang);
    itsyang=new char[itslen+1];
    for(unsigned short i=0;i<itslen; i++)
        itsyang[i]=cyang[i];
    itsyang[itslen]='\0';
}
yang::yang(const yang&rhs)
{
    cout <<"iiii\n";
    itslen=rhs.getlen();
    itsyang=new char[itslen+1];
    for(unsigned short i=0; i<itslen; i++)
        itsyang[i]=rhs[i];
    itsyang[itslen]='\0';
}
yang::~yang()
{
    delete[]itsyang;
    itslen=0;
}
yang& yang::operator=(const yang &rhs)
{
    if(this ==&rhs)
        return *this;     
    delete[]itsyang;  
    itslen=rhs.getlen();
    itsyang=new char[itslen+1];
    for(unsigned short i=0; i<itslen; i++)
        itsyang[i]=rhs[i];         
    itsyang[itslen]='\0';
    return *this;
}
char &yang::operator[](unsigned short offset)       
{
    if(offset>itslen)
    {
        return itsyang[itslen-1];
    }
    else
    {
        return itsyang[offset];
    }

}
char yang::operator[](unsigned short offset)const     
{
    if(offset>itslen)
    {
        return itsyang[itslen-1];
    }
    else
    {
        return itsyang[offset];
    }
}
yang yang::operator +(const yang&rhs)
{
    unsigned short totallen=itslen+rhs.getlen();
    yang temp(totallen);  
    unsigned short i;
    for(i=0; i<itslen; i++)
        temp[i]=itsyang[i];   
    for(unsigned short j=0;j<rhs.getlen(); j++,i++)
        temp[i]=rhs[j];       
    temp[totallen]='\0';        
    return temp;
}
void yang::operator+=(const yang&rhs)
{
    unsigned short rhslen=rhs.getlen();
    unsigned short totallen=itslen + rhslen;
    yang temp(totallen);   
    unsigned short i;
    for(i=0;i<itslen; i++)
        temp[i]=itsyang[i];  
    for(unsigned short j=0; j<rhs.getlen(); j++,i++)
        temp[i]=rhs[j];  
    temp[totallen]='\0';
    *this=temp;           
}
int main()
{
    yang s1("adfgfgdgdgdfgfda");
    cout << "s1：\t" << s1.getyang() << endl;
    char *temp ="bqdqdwqdqdqdqwdqqwdwqdqdwqdqdwqqwdqddwkkkkkbaaaa";          // 为什么这里超过39个字符就不输出cout << "s1：\t" << s1.getyang() << endl;
    cout << "s1：\t" << s1.getyang() << endl;
    char temptwo[20];
    strcpy(temptwo,"cc");
    s1+=temptwo;
    cout << "temptwo:\t" << temptwo << endl;

    cout << "s1:\t" << s1.getyang() << endl;
    cout << "s1[4]:\t" << s1[3] <<endl;

    s1[5]='x';
    cout << "s1：\t" << s1.getyang() << endl;

    cout << "s1[999];\t" << s1[999] << endl;

    yang s2("dd");
    yang s3;
    s3=s1+s2;               //这里为什么会调用复制函数  
    cout << "s3:\t" << s3.getyang() << endl;
    yang s4;
    s4 ="ee";
    cout << "s4:\t " << s4.getyang() << endl;

    return 0;
}