QueryResult.h

#ifndef __QUERYRESULT_H__
#define __QUERYRESULT_H__

#include <iostream>
#include <set>
#include <vector>
#include <string>
#include <memory>

class QueryResult{
  friend std::ostream& print(std::ostream&, const QueryResult&);
public:
  using line_no = std::vector<std::string>::size_type;
  using Iter = std::set<line_no>::iterator;
  QueryResult(std::string s, std::shared_ptr<std::set<line_no>> p,
              std::shared_ptr<std::vector<std::string>> f):
    sought(s), lines(p), file(f){}
    Iter begin() const {return lines->begin();}
    Iter end() const {return lines->end();}
    std::shared_ptr<std::vector<std::string>> get_file() const{
      return file;
    }
private:
  std::string sought;//查询的单词                                                    
  std::shared_ptr<std::set<line_no>> lines;//出现的行号                                   
  std::shared_ptr<std::vector<std::string>> file;
};
//QueryResult类的友元声明
std::ostream& print(std::ostream&, const QueryResult&);

#endif

TextQuery.h

#ifndef __TEXTQUERY_H__
#define __TEXTQUERY_H__

#include "QueryResult.h"
#include <map>
#include <iostream>
#include <fstream>
#include <sstream>
#include <set>
#include <vector>
#include <string>
#include <memory>

using namespace std;
class TextQuery{
 public:
  using line_no = std::vector<std::string>::size_type;

  TextQuery(ifstream& is);  
  QueryResult query(const std::string &sought) const;
 private:
  std::shared_ptr<std::vector<std::string>> file;
  std::map<std::string, std::shared_ptr<std::set<line_no>>> wm;
};

#endif

TextQuery.cpp

#include "TextQuery.h"

using namespace std;

TextQuery::TextQuery(ifstream& is) : file(new vector<string>){
  string text;
  while(getline(is, text)){//读文件的每一行                                   
    file->push_back(text);
    int n = file->size() - 1;//当前行号                                       
    istringstream line(text);//将行文本分解为单词                             
    string word;
    while(line >> word){
      //非常重要，必须用引用，要不然就会拷贝一个新的set给lines，不是原来的    
      auto &lines = wm[word];//lines是shared_ptr                              
      if(!lines)
    lines.reset(new set<line_no>);
      lines->insert(n);
    }
  }
}

QueryResult TextQuery::query(const string &sought) const{
  //如果没有找到sought，返回指向此set的一个智能指针                           
  static shared_ptr<set<line_no>> nodata(new set<line_no>);
  auto ret = wm.find(sought);
  if(ret == wm.end()){
    return QueryResult(sought, nodata, file);//没有找到                       
  }
  else{
    return QueryResult(sought, ret->second, file);
  }
}

main.cpp