当我在地图(C ++)中使用const_iterator时出现错误 [英] There is a error when I use the const_iterator in map (C++)

问题描述

大家好，我在使用map的const_iterator时遇到错误，下面是我的代码，请帮帮我，非常感谢！

Employ.cc

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

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

main.cc

main.cc

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

错误:

And the error:

<br />
<pre lang="vb">../src/main.cc:21: error: passing ‘const Employ’ as ‘this’ argument of ‘std::string Employ::getFirstName()’ discards qualifiers<br />
../src/main.cc:21: error: passing ‘const Employ’ as ‘this’ argument of ‘std::string Employ::getLastName()’ discards qualifiers</pre><br />

非常感谢！

Thank you very much!

推荐答案

xcnxx似乎有正确的答案-但让我解释一下.
首先-您遇到错误，因为您试图使用const_iterator来调用非const方法.显然，有两种解决方案:要么按照xcnxx的建议，使用常规迭代器，要么，如果我没有记错的话，这实际上更接近于您要执行的操作-将方法声明为const.为此，只需在每个方法的声明/定义末尾添加const关键字.参见此处的简单示例: wiki [

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

嗨！我的答案就在这里！

hi! my answer is here!

#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;

}

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