Problems with QList and const Class

Vinorcola

Hello, I have a problem when using QList<MyCLass*>::contains() and QList<MyClass*>::indexOf() with const instances.

Here is my example code :

@#ifndef MAINWINDOW_HPP
#define MAINWINDOW_HPP

#include <QList>
#include <QMainWindow>

#define ERROR -9

class MyClass
{
private:
QString m_name;

public:
    MyClass() :
        m_name("Default") {}

};

class MainWindow : public QMainWindow
{
Q_OBJECT

    QList<MyClass*> m_list;
    
public:
    MainWindow() :
        QMainWindow(),
        m_list() {}
    
    
    
    int getIndexOf(const MyClass* myClass) const
    {
        if (m_list.contains(myClass))
        {
            return m_list.indexOf(myClass);
        }
        else
        {
            return ERROR;
        }
    }

};

#endif // MAINWINDOW_HPP
@

When compiling, I have those errors:
MainWindow.hpp:40: erreur : invalid conversion from 'const MyClass*' to 'MyClass*' [-fpermissive]
qlist.h:886: error: initializing argument 1 of 'bool QList<T>::contains(const T&) const [with T = MyClass*]' [-fpermissive]
MainWindow.hpp:42: erreur : invalid conversion from 'const MyClass*' to 'MyClass*' [-fpermissive]
qlist.h:853: error: initializing argument 1 of 'int QList<T>::indexOf(const T&, int) const [with T = MyClass*]' [-fpermissive]

I do not realy understand why because those methodes are expected constant template :
QList::contains(const T&)
and here, T = MyClass* and I send a const MyClass* in parameter.

DerManu

There's a difference between a constant pointer to MyClass and a pointer to a constant MyClass. The getIndexOf Method has the latter kind as parameter, the QList methods expect the first kind.

Look:
int a;
someList.contains(a);
the "const T&" of QList::contains guarantees, that a won't be changed.

Similarly:
MyClass *a;
someList.contains(a);
guarantees, that a won't be changed. a, the pointer value (32/64-bit number). It's very different from const MyClass *a, which guarantees that *a, the object the 32/64-bit number points to, won't be changed.

Solution: try const_cast, or change the list type to const MyClass*

//EDIT: Something to think about along those lines:
@int i;
QList<const int> a;
QList<const int*> b;
a << i;
b << &i;
@
What will work, what will fail? Which list can/can't do what?

Vinorcola

Ok,
I understand the problem. Thanks for your help.