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c++iteratorerasequalifiers

When using map::erase() "error: passing 'const ***' as 'this' argument of '***' discards qualifiers [-fpermissive]" reported

I'm implementing the following method to delete an element from an associative table (map) with map::erase():

//Method in gestion.cpp
void Gestion::EliminateObject(string nomobjetg) const{

    auto it = objectname.find(nameobjectg);

    if (it == objectname.end())
        cout << "Object not found!" << endl;
    else
        objectname.erase(it); //The error is reported in this line
}

In the header I have defined the following:

//gestion.h
#include "objects.h"
#include <list>
#include <iostream>
#include <string>
#include <memory>
#include <map>
using namespace std;

typedef std::shared_ptr<Objects> ObjPtr;

typedef map<string, ObjPtr> Objectmap;

class Gestion
{
private:
     Objectmap objectname;

public:
    Gestion(Objectmap objectname, Groupmap groupname);
    virtual ~Gestion() {}
    virtual void EliminateObject(string nameobjectg) const;
};

Where ObjPtr is a virtual pointer of objects of type Objects.

When I compile I get this error:

error: passing ‘const Objectmap {aka const std::map<std::basic_string<char>, std::shared_ptr<Objects> >}’ as ‘this’ argument of ‘std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::erase(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator) [with _Key = std::basic_string<char>; _Tp = std::shared_ptr<Objects>; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, std::shared_ptr<Objects> > >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::basic_string<char>, std::shared_ptr<Objects> > >; std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const std::basic_string<char>, std::shared_ptr<Objects> > >]’ discards qualifiers [-fpermissive]
         objectname.erase(it);

Why is this line reporting this error? I've seen some other posts with this type of error but in those cases the root cause of the problem is related to how the method is declared (const). In this case the method involved in the problem is erase and I cannot modify it.

Solution

  • void Gestion::EliminateObject(string nomobjetg) const

    You are trying to modify objectname with your call to objectname.erase(it), and a const function will not let you do that, unless objectname is mutable.

    Remove const:

    void Gestion::EliminateObject(string nomobjetg)

    or

    Make objectname mutable

    mutable Objectmap objectname;