There is a error when I use the const_iterator in map (C++)

Question

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Hello,everyone,I have encountered an error when I use map's const_iterator,following are my codes,please help me,thank you very much!

Employ.cc

C#

#include <string>
#include <iostream>
using namespace std;
class Employ{
    friend class EmployCompare;
public:
    Employ(const string& firstname,const string& lastname):firstname_(firstname),lastname_(lastname){}
    string getFirstName(){return firstname_;}
    string getLastName(){return lastname_;}
private:
    string lastname_;
    string firstname_;
};
class EmployCompare{
public:
    bool operator()(const Employ& e1,const Employ e2) const{
        if(e1.lastname_ < e2.lastname_){
            return true;
        }
        else if(e1.lastname_ == e2.lastname_){
            return (e1.firstname_ < e2.firstname_);
        }
        else{
            return false;
        }
    }
};

main.cc

XML

#include <algorithm>
#include <iostream>
#include <iterator>
#include <map>
#include "Employ.cc"
using namespace std;
int main( )
{
    map<Employ,string,EmployCompare> employMap;
    Employ em1("lily","beautiful"),em2("boge","hardwork"),em3("relation","love"),em4("result","nice");
    employMap[em1] = "tester";
    employMap[em2] = "programmer";
    employMap[em3] = "coder";
    employMap[em4] = "student";
    map<Employ,string,EmployCompare>::const_iterator it = employMap.begin();
    while(it != employMap.end()){
        cout << it->first.getFirstName() << "--" << it->first.getLastName() << " is " << it->second << endl;
    }
    return 0;
}

And the error:

VB

../src/main.cc:21: error: passing ‘const Employ’ as ‘this’ argument of ‘std::string Employ::getFirstName()’ discards qualifiers
../src/main.cc:21: error: passing ‘const Employ’ as ‘this’ argument of ‘std::string Employ::getLastName()’ discards qualifiers

Thank you very much!

Posted 13-Jul-11 16:02pm

tiancehngbo

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tolw · Accepted Answer · 2011-07-13T18:32:00

xcnxx seems to have the right answer - but let me explain a little.

First of all - you encounter the errors because you are trying to use a const_iterator to call methods which are not const. Obviously there are two solutions: either, as xcnxx suggested, use a regular iterator, or, and if I'm not mistaken this is actually closer to what you intended to do - declare the methods as const. To do that simply add the const keyword at the end of each method's declaration/definition. See here for a simple example: wiki[^]

Please note however, that declaring the methods as const prohibits you from changing the object inside of the method - not a problem in this particular case, but something to bare in mind.

Good luck

xcnxx · Accepted Answer · 2011-07-13T17:06:00

Solution 1

hi! my answer is here!

XML

#include <iostream>
#include <vector>
#include "Employ.h"
using namespace std;

int main( )
{

    Employ em1("lily","beautiful"),em2("boge","hardwork"),em3("relation","love"),em4("result","nice");
    vector<Employ> vec;
    vec.push_back(em1);
    vec.push_back(em2);
    vec.push_back(em3);
    vec.push_back(em4);

    for(vector<Employ>::iterator it = vec.begin();it!=vec.end();it++)
    {
        cout << it->getFirstName() << "--" << it->getLastName() << endl;
    }
    return 0;
}

Posted 13-Jul-11 17:06pm

xcnxx

Comments

tiancehngbo 14-Jul-11 0:41am

thank you very much,your solution is ok,but what I want to try is using map,another person give me a sugesstion:
string getFirstName()const{return firstname_;}
string getLastName()const {return lastname_;}
and this works well,thank you all the same!

Philippe Mori 16-Jul-11 18:58pm

This is generally the preferred solution...

As soon as someone uses const, it must be used consistently...