QList with Struct issue in QT6

Ceraolo_gmail

I would like to use a QList of a struct, and in particular to use the "contains" function.
I have a nasty issue, that can be replicated using the following code:

struct STest{
    int i1,i2;
    bool operator== (const STest & x);
};
   QList <STest> myList;
    STest test1={1,2}, test2={11,12};

    myList.append(test1);
    myList.append(test2);

    bool contains;
    contains=myList.contains(test1);
    qDebug()<<contains;

It compiles without warnings and runs smoothly in Qt 5.15, but cannot compile in Qt6.4. I get the error message shown at the end of this post.
Despite of the many attempts I made I couls not sort this out.
Any suggestions? Thank you in advance.

D:\OneDrive - University of Pisa\Documenti\C++\Qt\untitled\mainwindow.cpp:28: error: passing 'const STest' as 'this' argument discards qualifiers [-fpermissive]
In file included from C:/Qt/6.4.0/mingw_64/include/QtCore/qobject.h:13,
                 from C:/Qt/6.4.0/mingw_64/include/QtWidgets/qwidget.h:9,
                 from C:/Qt/6.4.0/mingw_64/include/QtWidgets/qmainwindow.h:8,
                 from C:/Qt/6.4.0/mingw_64/include/QtWidgets/QMainWindow:1,
                 from ..\untitled\mainwindow.h:4,
                 from ..\untitled\mainwindow.cpp:1:
C:/Qt/6.4.0/mingw_64/include/QtCore/qlist.h: In instantiation of 'qsizetype QtPrivate::indexOf(const QList<T>&, const U&, qsizetype) [with V = STest; U = STest; qsizetype = long long int]':
C:/Qt/6.4.0/mingw_64/include/QtCore/qlist.h:932:30:   required from 'qsizetype QListSpecialMethodsBase<T>::indexOf(const AT&, qsizetype) const [with AT = STest; T = STest; qsizetype = long long int]'
C:/Qt/6.4.0/mingw_64/include/QtCore/qlist.h:44:31:   required from 'bool QListSpecialMethodsBase<T>::contains(const AT&) const [with AT = STest; T = STest]'
..\untitled\mainwindow.cpp:28:29:   required from here
C:/Qt/6.4.0/mingw_64/include/QtCore/qlist.h:903:20: error: passing 'const STest' as 'this' argument discards qualifiers [-fpermissive]
  903 |             if (*n == u)
      |                 ~~~^~~~
..\untitled\mainwindow.cpp:10:6: note:   in call to 'bool STest::operator==(const STest&)'
   10 | bool STest::operator== (const STest &x){
      |      ^~~~~

Christian Ehrlicher

The operator== must be const.

The compiler also tells you that you want to use a non-const function but expects a const one: passing 'const STest' as 'this' argument discards qualifiers [-fpermissive]

Ceraolo_gmail

@Christian-Ehrlicher Yes, the issue is somehow reated to the "constantness" of something.
But I don't understand how to solve this. To be acceptable by 5.15 the code requires 'const' in the parenthesis of the operator ==.

I even suspect that my example is correct since Qt 5.15 does not give any warnings or errors. In that case I would consider accceptable to bypass the Qt6 error message, but I don't know how to do this.
Maybe setting "-fpermissive"? But how to do this?

JonB

@Ceraolo_gmail
You are supposed to change your == operator declaration ~~to match exactly the one in the docs,~~ so:

bool operator== (const STest & x) const;

It is the const at the end that you are missing.

Christian Ehrlicher

This post is deleted!

Ceraolo_gmail

@JonB
Great! I could not find the piece of docs you linked.
This solves my issue.
Thanks a lot!

Christian Ehrlicher

You're right - the Qt documentation is not that clear about the constness but c++ is: https://en.cppreference.com/w/cpp/language/operator_comparison