利用cstring构造String类和其操作

关键点：

（1）operator+操作完成，会调用String::String(const  String & rhs)构造函数；

（2）引用返回问题：千万不要返回局部对象的引用。当函数执行完毕时，将释放分配给局部对象的存储空间。此时，对局部对象的引用就会指向不确定的内存。

       返回非引用类型：函数的返回值用于初始化在跳用函数出创建的临时对象。用函数返回值初始化临时对象与用实参初始化形参的方法是一样 的。  如果返回类型不是引用，在调用函数的地方会将函数返回值复制给临时对象。且其返回值既可以是局部对象，也可以是求解表达式的结果。

      返回引用：当函数返回引用类型时，没有复制返回值。相反，返回的是对象本身。

#include <iostream>
#include <cstring>

using namespace std;
class String
{
public:
String();
String(const char *const);
String(const String &);
~String();

char&  operator[] (unsigned short offset);
char   operator[] (unsigned short offset)const;

String operator+(const String&);
void   operator+=(const String&);
String& operator= (const String &);
unsigned short GetLen()const    { return itsLen;   }
const char*    GetString()const { return itsString;}
private:
String  (unsigned short);
char*   itsString;
unsigned short itsLen;
};

String::String()
{
itsString = new char[1];
itsString[0] = '\0';
itsLen=0;
cout<<"String() has been called! "<<endl;
}
String::String(unsigned short len)  //这个干什么用？
{
itsString = new char[len+1];
for (unsigned short i =0;i<=len;i++)
itsString[i] = '\0';
itsLen=len;
cout<<"String::String(unsigned short len) has been called! "<<endl;
}

String::String(const char* const cString)  //常量字符串调用初始化
{
itsLen = strlen(cString);
itsString = new char[itsLen+1];
for (unsigned short i=0;i<itsLen;i++)
itsString[i] = cString[i];
itsString[itsLen] = '\0';
cout<<"String::String(const char* const cString) has been called! "<<endl;
}

String::String(const String & rhs)  //
{
itsLen = rhs.GetLen();
itsString = new char[itsLen+1];
for (unsigned short i = 0;i<itsLen;i++)
itsString[i] = rhs[i];
itsString[itsLen] = '\0';

cout<<"String::String(const String & rhs) has been called! "<<endl;
cout<<"output is "<< itsString<<endl;
}
String::~String()
{
delete [] itsString;
itsLen = 0;
}
String& String::operator =(const String& rhs)
{
if (this == &rhs)
return *this;		//自身赋值
delete [] itsString;
itsLen=rhs.GetLen();
itsString = new char[itsLen+1];
for (unsigned short i = 0;i < itsLen;i++)
itsString[i] = rhs[i];
itsString[itsLen] = '\0';
cout<<" operater = has been called! "<<endl;
return *this;
}
// char& String::operator [](unsigned short offset)
// char String::operator [](unsigned short offset)const  区别？
char& String::operator [](unsigned short offset)
{
if (offset > itsLen)
return itsString[itsLen-1];
else
return itsString[offset];
}
char String::operator [](unsigned short offset)const
{
if (offset > itsLen)
return itsString[itsLen-1];
else
return itsString[offset];
}
String String::operator +(const String& rhs)
{
unsigned short totalLen = itsLen + rhs.GetLen();
String temp(totalLen);
unsigned short i;
for (i=0;i<itsLen;i++)
temp[i] = itsString[i];
for (unsigned short j = 0;j<rhs.GetLen();j++,i++)
temp[i] = rhs[j];
temp[totalLen] = '\0';
cout<<"String String::operator +(const String& rhs) has been called! "<<endl;
return temp;   //返回结果后，调用String::String(const String & rhs)
}

void String::operator +=(const String& rhs)
{
unsigned short rhsLen = rhs.GetLen();
unsigned short totalLen = itsLen + rhsLen;
String temp(totalLen);
unsigned short i;
for (i = 0;i<itsLen;i++)
temp[i] = itsString[i];
for (unsigned short j = 0;j<rhs.GetLen();j++,i++)
temp[i] = rhs[i-itsLen];
temp[totalLen] = '\0';
*this = temp;
}
int main()
{

String s1("initial test");
//String s(s1);
/*
cout<<"S1:/t"<<s1.GetString()<<endl;
char *temp ="Hello World";
s1 = temp;
cout<<"S1:/t"<<s1.GetString()<<endl;
char tempTwo[20];
strcpy(tempTwo,"; nice to be here!");
cout<<" s1:/t "<<s1.GetString()<<endl;

s1 += tempTwo;
cout<<"tempTwo:/t "<<tempTwo<<endl;

cout<<"S1:/t "<<s1.GetString()<<endl;
cout<<"S1 size is "<<s1.GetLen()<<endl;

cout<<"S1[4]:/t "<<s1[4]<<endl;
cout<<"S1[999]:/t "<<s1[999]<<endl;  //越界没有问题？？
*/
String s2(" Anoter string");
//cout<<"s1 + s2 = "<<(s1+s2).GetString()<<endl;
s1+s2;
cout<<endl<<endl;

String s3;
s3 = s1+s2;  //s1 + s2 后生成一个零时变量
cout<<"S3:/t " <<s3.GetString()<<endl;

String s4;
s4 = "Why does this work?";
cout<<"S4:/t "<<s4.GetString()<<endl;
cout<<"'why are you ? '"<<"has "<<sizeof("why are you ?")<<"  characters! "<<endl;
cout<<"'why are you ? '"<<"has "<<strlen("why are you ?")<<"  characters! "<<endl;
return 0;
}