文件main.cc

#include <string>
#include <iostream>
#include <memory>
#include <vector>
#include <set>
#include <map>
#include <fstream>
#include "resultquery.h"
#include "textquery.h"
using namespace std;
int main(int number,char**para)
{
ifstream in_f(para[1]);
runQueries(in_f);
return 0;
}

文件functions.cc

#include <string>
#include <iostream>
#include <memory>
#include <vector>
#include <set>
#include <map>
#include "resultquery.h"
#include "textquery.h"
#include <sstream>
#include <fstream>
using namespace std;
TextQuery::TextQuery(ifstream &in):file(new vector<string>()){
string line;
size_t line_no = 0;while(getline(in,line)){file->push_back(line);string word;istringstream is(line);while(is >> word){if(!str_lines[word])str_lines[word] = make_shared<set<size_t>>();str_lines[word]->insert(line_no);}line_no ++; }
}
ResultQuery TextQuery::query(const string &s)const{
static  shared_ptr<set<size_t>> nodata(new set<size_t>());
if(str_lines.find(s) == str_lines.end())
{return ResultQuery(file,nodata,s);
}
else {//错误点已经找到：
//把这个块内的代码改成下面的格式就正确了，为啥？？？
//  map<string,shared_ptr<set<size_t>>>::const_iterator pos = str_lines.find(s);
//       return ResultQuery(file,pos->second,s);return ResultQuery(file,str_lines[s],s);
//现在的函数是const的，所以这个成员也变成了const的str_lines为啥？
//成员函数是const的，那么会导致调用这个成员函数的所有成员不可改变,就说每个成员都是const的了
//所以就不能用下标运算了，map的下标运算只适合非const的map对象}}ResultQuery::ResultQuery():file(new vector<string>),lines(new set<size_t>){}
ResultQuery::ResultQuery(shared_ptr<vector<string>> f,shared_ptr<set<size_t>> l,const string &w):file(f),lines(l),word(w){}set<size_t>::iterator ResultQuery::begin(){return lines->begin();
}
set<size_t>::iterator ResultQuery::end(){return lines->end();
}
shared_ptr<vector<string>>
ResultQuery::get_file(){return file;}void runQueries(ifstream &in){
TextQuery t(in);
string word;
cout << "enter a word you want to find(q to end):" << endl;
cin >> word;
print(cout,t.query(word));
}ostream& print(ostream &os,const ResultQuery &qr)
{
os << qr.word << " occurs " << qr.lines->size() << " times:" << endl;
for(auto &i : *qr.lines){cout << "(line " << i+1 << " )" << (*qr.file)[i] << endl;}
return os;
}

文件testquery.h

#ifndef TEXTQUERY_H
#define TEXTQUERY_H#include <string>
#include <iostream>
#include <memory>
#include <vector>
#include <set>
#include <map>
#include "resultquery.h"
using namespace std;
class ResultQuery;
class TextQuery{public:TextQuery():file(make_shared<vector<string>>()){}TextQuery(ifstream &);ResultQuery query(const string &)const;private:shared_ptr<vector<string>> file;map<string,shared_ptr<set<size_t>>> str_lines;};void runQueries(ifstream &);#endif

文件resultquery.h

#ifndef RESULTQUERY_H
#define RESULTQUERY_H
#include <string>
#include <iostream>
#include <memory>
#include <vector>
#include <set>
#include <map>
using namespace std;
class ResultQuery{friend    ostream& print(ostream &,const ResultQuery &qr);public:ResultQuery();ResultQuery(shared_ptr<vector<string>> f,shared_ptr<set<size_t>> l,const string &w);shared_ptr<vector<string>> get_file();set<size_t>::iteratorbegin();set<size_t>::iteratorend();private:shared_ptr<vector<string>> file;shared_ptr<set<size_t>> lines;string word;};
ostream & print(ostream &,const ResultQuery &qr);#endif

在ubuntu20.04中执行编译：

g++ main.cc functions.cc -o 123

结果如下：

functions.cc: In member function ‘ResultQuery TextQuery::query(const string&) const’:
functions.cc:38:43: error: passing ‘const
std::map<std::__cxx11::basic_string<char>, std::shared_ptr<std::set<long unsigned int> > >’ as ‘this’ argument discards qualifiers [-fpermissive]38 |        return ResultQuery(file,str_lines[s],s);|                                           ^
In file included from /usr/include/c++/9/map:61,from functions.cc:6:
/usr/include/c++/9/bits/stl_map.h:490:7: note:   in call to ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = std::shared_ptr<std::set<long unsigned int> >; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, std::shared_ptr<std::set<long unsigned int> > > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::shared_ptr<std::set<long unsigned int> >; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char>]’490 |       operator[](const key_type& __k)

为啥错误？

代码中的分析得到如下结论：

const成员函数的所有成员不可改变，就是说const成员函数把调用它的对象所有成员都const化了（即使原来不是const的成员，现在都不可改变就变成const的了）。如下：

//test.cc

class T{public:int *ptr()const{return &a;}private:int a;int b;};

编译后错误如下：

test.cc: In member function ‘int* T::ptr() const’:
test.cc:11:26: error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]11 |   int *ptr()const{return &a;}|                          ^~|                          ||                          const int*

也就是说现在成员函数中的a就是const的了，即使原来的类中不是const的。所以返回值&a就是const int*类型，当然不能转化成int *。

到此，开始代码中文件functions.cc中的错误就非常好理解了，const成员函数query中所有的数据成员都const化了，所以str_lines也是const的了，所以它的元素的指针当然是const的（find成员函数的到的指针也是const的），所以导致错误。所以const map对象不能用下标操作，但是可以用指针操作来访问值。

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error: passing ‘const xxx’ as ‘this’ argument discards qualifiers c++primer 5th文本查询程序一个错误请各位指教（已解决）

const成员函数的所有成员不可改变，就是说const成员函数把调用它的对象所有成员都const化了（即使原来不是const的成员，现在都不可改变就变成const的了）。如下：

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