里面一句代码看不懂.

//: C06:NString.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.
// A "numbered string" that keeps track of the
// number of occurrences of the word it contains.
#ifndef NSTRING_H
#define NSTRING_H
#include <algorithm>
#include <iostream>
#include <string>
#include <utility>
#include <vector>

typedef std::pair<std::string, int> psi;

// Only compare on the first element
bool operator==(const psi& l, const psi& r) {
  return l.first == r.first;
}

class NString {
  std::string s;
  int thisOccurrence;
  // Keep track of the number of occurrences:
  typedef std::vector<psi> vp;
  typedef vp::iterator vpit;
  static vp words;
  void addString(const std::string& x) {
    psi p(x, 0);
    vpit it = std::find(words.begin(), words.end(), p);
    if(it != words.end())
      thisOccurrence = ++it->second;
    else {
      thisOccurrence = 0;
      words.push_back(p);
    }
  }
public:
  NString() : thisOccurrence(0) {}
  NString(const std::string& x) : s(x) { addString(x); }
  NString(const char* x) : s(x) { addString(x); }
  // Implicit operator= and copy-constructor are OK here.
  friend std::ostream& operator<<(
    std::ostream& os, const NString& ns) {
    return os << ns.s << " [" << ns.thisOccurrence << "]";
  }
  // Need this for sorting. Notice it only
  // compares strings, not occurrences:
  friend bool
  operator<(const NString& l, const NString& r) {
    return l.s < r.s;
  }
  friend
  bool operator==(const NString& l, const NString& r) {
    return l.s == r.s;
  }
  // For sorting with greater<NString>:
  friend bool
  operator>(const NString& l, const NString& r) {
    return l.s > r.s;
  }
  // To get at the string directly:
  operator const std::string&() const { return s; }//请问这句是什么意思?
};

NString::vp NString::words;   ///////请问这句代码是什么意思啊?
#endif // NSTRING_H ///:~


[ 本帖最后由 q1826050194 于 2012-7-14 14:34 编辑 ]