关于c＋＋primer中的一个类类型String的问题。

我在Vc＋＋6。0，dev－c＋＋，gcc for windows中都编译过了，就是通不过。
我按照书上的源码如下，帮忙看看有什么问题没有啊：
#include<iostream>
#include<cstring>
#include<cassert>
#include<iomanip>
using namespace std;

class String;
istream& operator>>(istream&,String&);
ostream& operator<<(ostream&,const String&);

class String{
public:
String();
String(const char*);
String(const String&);

~String();

String& operator=(const String&);
String& operator=(const char*);

bool operator==(const String&);
bool operator==(const char*);

char& operator[](int);

int size(){return _size;}
char* c_str(){return _string;}

private:
int _size;
char *_string;
}

inline String::String()
{
_size=0;
_string=0;
}

inline String::String(const char*str)
{
if(!str)
{
_size=0;
_string=0;
}
else
{
_size=strlen(str);
_string=new char[_size+1];
strcpy(_string,str);
}
}


inline String::String(const String &rhs)
{
_size=rhs._size;
if(!rhs._string)
_string=0;
else
{
_string=new char[_size+1];
strcpy(_string,rhs._string);
}
}

inline String::~String(){delete [] _string;}


inline String&
String::operator=(const char *s)
{
if(!s)
{
_size=0;
delete [] _string;
_string=0;
}
else
{
_size=strlen(s);
delete [] _string;
_string=new char[_size+1];
strcpy(_string,s);
}

return *this;
}

inline String&
String::operator=(const String &rhs)
{
if(this!=&rhs){
delete [] _string;
_size=rhs._size;

if(!rhs._string) _string=0;
else{
_string=new char[_size+1];
strcpy(_string,rhs._string);
}
}
return *this;
}

inline bool
String::operator==(const String &rhs)
{
if(_size!=rhs._size)
return false;
return strcmp(_string,rhs._string)?false:true;
}

inline bool
String::operator==(const char*s)
{
return strcmp(_string,s)?false:true;
}

inline char&
String::operator[](int elem)
{
assert(elem>0&&elem<_size);
return _string[elem];
}


inline istream&
operator>>(istream &io,String &s)
{
const int limit_string_size=4096;
char inBuf[limit_string_size];

io>>setw(limit_string_size)>>inBuf;

s=inBuf;
return io;
}

inline ostream&
operator<<(ostream& os,String &s)
{
return os<<s.c_str();
}


int main()
{
int aCnt=0,eCnt=0,iCnt=0,oCnt=0,uCnt=0,theCnt=0,itCnt=0,wdCnt=0,notVowel=0;

String buf,the("the"),it("it");

while(cin>>buf){
++wdCnt;

cout<<buf<<' ';
if(wdCnt%12==0)
cout<<endl;

if(buf==the||buf=="The")
++theCnt;
else
if(buf==it||buf=="It")
++itCnt;

for(int ix=0;ix<buf.size();++ix)
{
switch(buf[ix])
{
case 'a':case 'A':++aCnt;break;
case 'e':case 'E':++eCnt;break;
case 'i':case 'I':++iCnt;break;
case 'o':case 'O':++oCnt;break;
case 'u':case 'U':++uCnt;break;
default:++notVowel;break;
}
}
}

cout<<"\n\n"
<<"Word read: "<<wdCnt<<"\n\n"
<<"the/The: "<<theCnt<<'\n'
<<"it/It: "<<itCnt<<"\n\n"
<<"not-vowels read: "<<notVowel<<"\n\n"
<<"a: "<<aCnt<<'\n'
<<"e: "<<eCnt<<'\n'
<<"i: "<<iCnt<<'\n'
<<"o: "<<oCnt<<'\n'
<<"u: "<<uCnt<<endl;
return 0;
}