QListSpecialMethodsBase (QList and QMap) .contains results in error passing as ‘this’ argument discards qualifiers [-fpermissive]

squinky86

I started porting to Qt6. The issue I'm having is with every time I use the "contains()" function on a map or list. Using this line as an example, the error I get is:

g++ -c -pipe -O2 -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_SQL_LIB -DQT_XML_LIB -DQT_CORE_LIB -I. -Isrc -I/opt/qt6.3-static/include -I/opt/qt6.3-static/include/QtWidgets -I/opt/qt6.3-static/include/QtGui -I/opt/qt6.3-static/include/QtNetwork -I/opt/qt6.3-static/include/QtSql -I/opt/qt6.3-static/include/QtXml -I/opt/qt6.3-static/include/QtCore -I. -I. -I/opt/qt6.3-static/mkspecs/linux-g++ -o workerassetdelete.o src/workerassetdelete.cpp
In file included from /opt/qt6.3-static/include/QtCore/qvector.h:43,
                 from /opt/qt6.3-static/include/QtCore/QVector:1,
                 from src/dbmanager.h:25,
                 from src/workerassetdelete.cpp:20:
/opt/qt6.3-static/include/QtCore/qlist.h: In instantiation of ‘qsizetype QtPrivate::indexOf(const QList<T>&, const U&, qsizetype) [with V = Asset; U = Asset; qsizetype = long long int]’:
/opt/qt6.3-static/include/QtCore/qlist.h:968:30:   required from ‘qsizetype QListSpecialMethodsBase<T>::indexOf(const AT&, qsizetype) const [with AT = Asset; T = Asset; qsizetype = long long int]’
/opt/qt6.3-static/include/QtCore/qlist.h:80:31:   required from ‘bool QListSpecialMethodsBase<T>::contains(const AT&) const [with AT = Asset; T = Asset]’
src/workerassetdelete.cpp:86:37:   required from here
/opt/qt6.3-static/include/QtCore/qlist.h:939:20: error: passing ‘const Asset’ as ‘this’ argument discards qualifiers [-fpermissive]
  939 |             if (*n == u)
      |                 ~~~^~~~
In file included from src/dbmanager.h:29,
                 from src/workerassetdelete.cpp:20:
src/asset.h:78:10: note:   in call to ‘bool Asset::operator==(const Asset&)’
   78 |     bool operator==(const Asset &right);
      |          ^~~~~~~~
make: *** [Makefile:6555: workerassetdelete.o] Error 1

In searching through these forums, I found a lot of information about serialization, but I can't find any examples of how to call contains() correctly with Qt6.
Thanks!

squinky86

This post is deleted!

Christian Ehrlicher

We can't really help without seeing your code which triggers this warning.

squinky86

This post is deleted!

Christian Ehrlicher

bool operator==(const Asset &right);

this function must be const as explained here:

/opt/qt6.3-static/include/QtCore/qlist.h:939:20: error: passing ‘const Asset’ as ‘this’ argument discards qualifiers [-fpermissive]
939 | if (*n == u)
| ~^~
In file included from src/dbmanager.h:29,
from src/workerassetdelete.cpp:20:
src/asset.h:78:10: note: in call to ‘bool Asset::operator==(const Asset&)’
78 | bool operator==(const Asset &right);

artwaw

@squinky86 said in QListSpecialMethodsBase (QList and QMap) .contains results in error passing as ‘this’ argument discards qualifiers [-fpermissive]:

this line as an example

you have anitpattern here

if (toDelete.keys().contains(a))

according to this https://www.kdab.com/uncovering-32-qt-best-practices-compile-time-clazy/ there should be simply

toDelete.contains(a)

Also, Q_FOREACH is rather deprecated and overall performance poor https://www.kdab.com/goodbye-q_foreach/

I'd fix those first and then see if the error persists.

squinky86

@artwaw Thanks! You are correct - these are good code cleanups to implement.

squinky86

@Christian-Ehrlicher I'm sorry - I clicked on the wrong menu - this is the correct answer and resolved the compile issue. I can't figure out how to go back and mark this as the answer.

Christian Ehrlicher

Hehe, no problem :)