About a stange issue with a map variable inside a const function

[tonnot]

I have a map<int<map<int,char> > variable.
It is declared at the private section of my .h file :

map<unsigned int,std::map<unsigned int,char> > map__type;
I also have :
vector<string> vecHheader;

I use it in this function with sufix const keyword
myfunction (bla bla) const

Ok I can return vector information but not map ???
( I have the const error : ... bla bla bla dicard qualifiers.)

Whats happen ? I can not use map into a const functions ? I want only to returning information !

Any idea ? Thanks .


And , If I made mymap mutable... What can I expect from mymap or const function perfomance ?

[tonnot]

I'm trying to use map::iterator code I found on the web but... code completion no offers me ->first neither ->second ....

And when compile I have an error and recomendation to use _rb_tree_iterator?

And... if I use std::map at .h file I have not completion code for maps and vectors, I have to use 'using namespace std'.

Any idea ?
(I'm using QT+mingw)

[wysota]

Sorry but what are you asking about? Constness is constness, regardless of types/classes you use.

[ChrisW67]

Qt does not stop you using any other part of C++, including the STL. I have found the code completion in Qt Creator to be a little hit and miss with the STL iterator constructs though (.first and .second have never worked for me).

The namespace stuff is generic C++. Including <map> defines a map class in the std namespace. You can access that explicitly as std::map. If you wish, you may make the entire content of the std namespace available in global scope with "using namespace std;". Then you can access std::map as just map.

What does the code that generates the error and the error itself look like?

[tonnot]

I'm asking for the quesion :
'I can return vector information but not map ???
(the function in which I use this is a reimplemented QVariant() tablemodel::data() const
Thanks

[tonnot]

I have :

map<char,int> mymap;
      map<char,int>::iterator it;
 
      mymap['a']=50;
      mymap['b']=100;
      mymap['c']=150;
      mymap['d']=200;
 
      it=mymap.find('b');
      cout<<it->second<< endl;

map<char,int> mymap;
      map<char,int>::iterator it;

      mymap['a']=50;
      mymap['b']=100;
      mymap['c']=150;
      mymap['d']=200;

      it=mymap.find('b');
      cout<<it->second<< endl;

To copy to clipboard, switch view to plain text mode 

It works at the constructor of my custom tablemodel constructor but not at the reimplemented function mycustomtablemodel::data () const

An this is the error and the recomendation to use _rb_tree....

error: no match for 'operator=' in '((const A_model*)this)->A_model::it = ((const A_model*)this)->A_model::mymap.std::map<_Key, _Tp, _Compare, _Alloc>::find [with _Key = char, _Tp = int, _Compare = std::less<char> ....
note: candidate is: std::_Rb_tree_iterator<std:air<const char, int> >& std::_Rb_tree_iterator<std:air<const char, int> >:perator=(const std::_Rb_tree_iterator<std:air<const char, int> >&)


Simply I cannot use 'map' code I found on the web.
I am at dead point with this....


(Something happens with colon colon p , forum message imagine that it is an smile... and it is colon colon pair )

[high_flyer]

What happens if you change the name of mymap to something else?

[tonnot]

I dont understand you. ( change the name of mymap to something else? )

I have fixed the problem with a poor solutiuon: make my maps mutable, and use mymaps[][].
It is a bad approach, but I dont know how to use the rb_tree_ and ... I dont understand why it happens with maps and however I can use list, vectors, etc inside this const function....

Any more ideas ? Thanls

[high_flyer]

I dont understand you. ( change the name of mymap to something else? )

I thought there might be some type conflicst since the error says you are using 'const A_model*' while in your code you are using char and int.

I have fixed the problem with a poor solutiuon: make my maps mutable, and use mymaps[][].

That means you have a 'const' problem.
Show the real code you are using.

[tonnot]

Thanks for your time HF
This is the complete .h file and the data function of .cpp. ( a piece o cpp file, becasue it is full of comments and )
As I said, now (with the mutable change) it works, but I dont understand the rb_tree advise if I dont use mutable.
(And also I dont understand why I can use vector without problems inside this const functions)

A_model.h

#include <QtGui/QTableView>
#include <QtGui/QHeaderView>
#include <QAbstractTableModel>
#include <QItemSelection>
#include <QObject>
#include <string>
#include <map>
#include <vector>
using namespace std;
 
class A_model : public QAbstractTableModel
{
     Q_OBJECT
 
public:
    A_model(int rows, int cols, QObject *parent=0);
 
    //reimplemented
    int rowCount(const QModelIndex &parent) const;
    int columnCount(const QModelIndex &parent) const;
    QVariant      data(const QModelIndex &index, int role) const;
    QVariant      headerData(int section, Qt::Orientation orientation, int role) const;
    virtual bool  setHeaderData(int section,Qt::Orientation orientation,const QVariant &value);
    Qt::ItemFlags flags(const QModelIndex &index) const;
    bool          setData(const QModelIndex &index, const QVariant &value, int role=Qt::EditRole);
    bool          insertRows(int position, int rows, const QModelIndex &index=QModelIndex());
    bool          removeRows(int position, int rows, const QModelIndex &index=QModelIndex());
    bool          insertColumns(int column, int count, const QModelIndex &index);
 
   // custom functions
    void set_cols(int cols) ;
    void set_rows(int rows) ;
    int M_ROWS, M_COLS, M_ROW, M_COL;
 
private:
 
    QList< QPair<QString, QString> > listOfPairs;
    vector<int> i_vector;         //1
    vector<long> l_vector;        //2
    vector<float> f_vector;       //3
    vector<double> d_vector;      //4
    vector<std::string> s_vector; //5
 
   mutable map<unsigned int,map<unsigned int,char> > map__type;
   mutable map<unsigned int,map<unsigned int,unsigned int> > map_index;
 
   mutable map<unsigned int,map<unsigned int,char> >::iterator map__type_i1;
   mutable map<unsigned int,char>::iterator map__type_i2;
 
 
 
    vector<string> vecHheader;
    vector<string> vecVheader;
};

#include <QtGui/QTableView>
#include <QtGui/QHeaderView>
#include <QAbstractTableModel>
#include <QItemSelection>
#include <QObject>
#include <string>
#include <map>
#include <vector>
using namespace std;

class A_model : public QAbstractTableModel
{
     Q_OBJECT

public:
    A_model(int rows, int cols, QObject *parent=0);

    //reimplemented
    int rowCount(const QModelIndex &parent) const;
    int columnCount(const QModelIndex &parent) const;
    QVariant      data(const QModelIndex &index, int role) const;
    QVariant      headerData(int section, Qt::Orientation orientation, int role) const;
    virtual bool  setHeaderData(int section,Qt::Orientation orientation,const QVariant &value);
    Qt::ItemFlags flags(const QModelIndex &index) const;
    bool          setData(const QModelIndex &index, const QVariant &value, int role=Qt::EditRole);
    bool          insertRows(int position, int rows, const QModelIndex &index=QModelIndex());
    bool          removeRows(int position, int rows, const QModelIndex &index=QModelIndex());
    bool          insertColumns(int column, int count, const QModelIndex &index);

   // custom functions
    void set_cols(int cols) ;
    void set_rows(int rows) ;
    int M_ROWS, M_COLS, M_ROW, M_COL;

private:

    QList< QPair<QString, QString> > listOfPairs;
    vector<int> i_vector;         //1
    vector<long> l_vector;        //2
    vector<float> f_vector;       //3
    vector<double> d_vector;      //4
    vector<std::string> s_vector; //5

   mutable map<unsigned int,map<unsigned int,char> > map__type;
   mutable map<unsigned int,map<unsigned int,unsigned int> > map_index;

   mutable map<unsigned int,map<unsigned int,char> >::iterator map__type_i1;
   mutable map<unsigned int,char>::iterator map__type_i2;



    vector<string> vecHheader;
    vector<string> vecVheader;
};

To copy to clipboard, switch view to plain text mode 

And the cpp.

QVariant A_model::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
    {         return QVariant();}
 
    if (index.row() >= M_ROWS || index.row() < 0)
       { return QVariant();}
 
    if (role == Qt::DisplayRole) {
 
     unsigned int rr=(unsigned int)(index.row());
     unsigned int cc=(unsigned int)(index.column());
            switch (map__type[rr][cc])
           {
           case 1:
               break;
           case 2:
               break;
           case 3:
               break;
           case 4:
               break;
           case 5:  
               return W_utiles().To_qstring(map_index[rr][cc])+" "+ QString().fromStdString(s_vector.at(map_index[rr][cc]));
               break;
            }
}

QVariant A_model::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
    {         return QVariant();}

    if (index.row() >= M_ROWS || index.row() < 0)
       { return QVariant();}

    if (role == Qt::DisplayRole) {
  
     unsigned int rr=(unsigned int)(index.row());
     unsigned int cc=(unsigned int)(index.column());
            switch (map__type[rr][cc])
           {
           case 1:
               break;
           case 2:
               break;
           case 3:
               break;
           case 4:
               break;
           case 5:  
               return W_utiles().To_qstring(map_index[rr][cc])+" "+ QString().fromStdString(s_vector.at(map_index[rr][cc]));
               break;
            }
}

To copy to clipboard, switch view to plain text mode 

I use a vector so have the data for my headers. And can use it at this function without problem

QVariant A_model::headerData(int section, Qt::Orientation orientation, int role) const
....
return  vecHheader[section];
.....

QVariant A_model::headerData(int section, Qt::Orientation orientation, int role) const
....
return  vecHheader[section];
.....

To copy to clipboard, switch view to plain text mode 

(This is because I say I can use vector without problems but not map)

Help.

[high_flyer]

Two things:
1. As usual, I read your post to quickly, but the answer is there if one reads carefully:

error: no match for 'operator=' in '((const A_model*)this)->A_model::it = ((const A_model*)this)->A_model::mymap.std::map<_Key, _Tp, _Compare, _Alloc>::find [with _Key = char, _Tp = int, _Compare = std::less<char> ....
note: candidate is: std::_Rb_tree_iterator<std:air<const char, int> >& std::_Rb_tree_iterator<std:air<const char, int> >erator=(const std::_Rb_tree_iterator<std:air<const char, int> >&)

Now you using mutable got me on the right track - namely, const problem.

Which has nothing to do with STL, but with your funciton definition:

QVariant A_model::headerData(int section, Qt::Orientation orientation, int role) const

'const' means your method does not alter the content of the class, but you ARE trying to.
That is the reason your iterator complains, he expects const types, where as you gave it char and not const char.

If you remove the const from your method definition/declaration, the error should go away.

That is also the reason your vector works, since you are just returning , not altering, so there your method is correctly const

[tonnot]

But I want to use map_index[rr][cc]) to return data.....

[high_flyer]

So?
Did you read (and understand) my last post?

[tonnot]

You say :

'const' means your method does not alter the content of the class, but you ARE trying to.

Ok, I understand but I only wnat to return data when the tablemodel is asking me for it.
I see a lot of examples using vectors, lists, Qlists, etc
And the question is ,MAP has a different behavior and MAP[][] is interpretated as a change ?
Thanks and thanks for your patience.

[high_flyer]

I see.
But you also have the syntax semantics that you need to respect, and std::map is a bit special too.
Since a const method is not allowed to change things, using non const expressions is not allowed.
Since your map is defined with non const types, you get a problem.
In the case of std::map however, you really can't avoid this, since the operator[] creates a new entry if the entry does not exist, and it would not be able to do it, if it was const.
So in this case your solution is to define your method not as const - OR - use sts::map::find() instead of [].

[tonnot]

Ok, and thank you very much