类中常量定义出错，该怎么解决呢

天泪离星

书本上的例程，但是我按照书上的敲上去结果却不对……
//这个是头文件
#ifndef STRING1_H_
#define STRING1_H_
#include <iostream>
using std::ostream;
using std::istream;
class String
{
private:
char *str;
int len;
static int num_strings;
static const int CINLIM = 80;
public:
String(const char *s);
String();
String(const String &);
~String();
int length()const{return len;}
String & operator=(const String &);
String & operator=(const char *);
char & operator[](int i);
const char & operator[](int i) const;
friend bool operator<(const String &st,const String & st2);
friend bool operator>(const String &st,const String & st2);
friend bool operator==(const String &st,const String & st2);
friend ostream & operator<<(ostream & os,const String & st);
friend istream & operator>>(istream & is,const String & st);
static int HowMany();
};
#endif
//这个是类的实现文件
#include <cstring>
#include "string1.h"
using std::cin;
using std::cout;
int String::num_strings=0;
int String::HowMany()
{
return num_strings;
}
String::String(const char *s)
{
len=strlen(s);
str=new char[len+1];
strcpy(str,s);
num_strings++;
}
String::String()
{
len =4;
str=new char[1];
str[0]='\0';
num_strings++;
}
String::String(const String &st)
{
num_strings++;
len=st.len;
str=new char[len+1];
strcpy(str,st.str);
}
String::~String()
{
--num_strings;
delete []str;
}
Strin