Qt6.2.1 QList index out of bound

sbela

This throws an index out of bounds for me ! But I can not figure out why!
Maybe this:
Q_ASSERT_X(size_t(i) + size_t(n) <= size_t(d->size), "QList::remove", "index out of range");
shoud be this
Q_ASSERT_X(size_t(i) + size_t(n) < size_t(d->size), "QList::remove", "index out of range");

#include <QCoreApplication>
#include <iostream>

class A
{
public:
    explicit A(QString name) : m_name(name) { }

    bool operator==(const A &other) const
    {
        return other.m_name == m_name;
    }
private:
    QString m_name;
};

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
    A a("ONE");
    QList<A> a_list;

    a_list.append(a);

    std::cout << "Item count:" << a_list.count() << std::endl;

    if (auto index = a_list.indexOf(A("ONE")) > -1)
    {
        std::cout << "Item index:" << index << std::endl;
        a_list.removeAt(index);
    }

    return app.exec();
}

Christian Ehrlicher

If you want to use c++17 you should learn the syntax...

if (auto index = a_list.indexOf("ONE"); index > -1)

jsulm

@sbela said in Qt6.2.1 QList index out of bound:

std::cout << "Item index:" << index << std::endl;

What does this print out?

sbela

This is the output:

16:33:31: Starting /home/.../work/projects/build-qlistqt6test-Desktop_Qt_6_2_1_GCC_64bit-Debug/qlistqt6test...
Item count:1
Item index:1
ASSERT failure in QList::remove: "index out of range", file /opt/Qt5.15.0/6.2.1/gcc_64/include/QtCore/qlist.h, line 741
16:33:31: /home/.../work/projects/build-qlistqt6test-Desktop_Qt_6_2_1_GCC_64bit-Debug/qlistqt6test crashed.

jsulm

@sbela said in Qt6.2.1 QList index out of bound:

Item index:1

Then you're indeed out of bounds.
Just not sure why indexOf returns 1 instead of 0

KroMignon

@sbela I have many problems with your code. Does this really build?

AFAIK, to use QList you need a default constructor. So I would change explicit A(QString name) : m_name(name) { } to explicit A(const QString &name = QString()) : m_name(name) { }.

Second, I may be wrong, but for my understanding if (auto index = a_list.indexOf(A("ONE")) > -1) will create a local variable index which is destroyed after the test.
And the calculation is also wrong your have done index = a_list.indexOf(A("ONE")) > -1, which should be (index = a_list.indexOf(A("ONE"))) > -1

I would change this to:

int index;
    if ((index = a_list.indexOf(A("ONE"))) > -1)
    {
        std::cout << "Item index:" << index << std::endl;
        a_list.removeAt(index);
    }

JonB

@sbela said in Qt6.2.1 QList index out of bound:

if (auto index = a_list.indexOf(A("ONE")) > -1)

Oh dear! And this is why index is being set to 1 @jsulm

Hint: Operator precedence.... :)

P.S.
@KroMignon
Your post crossed with mine.

Second, I may be wrong, but for my understanding if (auto index = a_list.indexOf(A("ONE")) > -1) will create a local variable index which is destroyed after the test.

No. Says in scope for whole of if body. New (well not very new) C++ thingie :)

which should be (index = a_list.indexOf(A("ONE"))) > -1

Indeed! But you gave it away! ;-) I assume the whole line with the auto would actually read:

if ((auto index = a_list.indexOf(A("ONE"))) > -1)

jsulm

@JonB Good catch!

sbela

Ok! But this:

if ((auto index = a_list.indexOf(A("ONE"))) > -1)

does not compile for me because:

error: expected primary-expression before ‘auto’
   52 |     if ((auto index = a_list.indexOf(A("ONE"))) > -1)
      |          ^~~~

@JonB said in Qt6.2.1 QList index out of bound:

if ((auto index = a_list.indexOf(A("ONE"))) > -1)

JonB

@sbela
It was only a guess, because I would never use auto, and not in the condition of an if!

Can it be:

 if (auto (index = a_list.indexOf(A("ONE"))) > -1)

? I can't believe that's right!

In which case, you tell me how you can both use auto (or explicit type specifier) and get the operator precedence right here, because it's not clicking with me :) And I'm not sure how to Google for this....

P.S.
Did I recently see yet another C++-ism which would allow this:

 if (auto index; (index = a_list.indexOf(A("ONE"))) > -1)

? See that ; and multi-statement in the condition? I believe that's a "thing" now with C++, might require a recent version? @jsulm , @KroMignon ?

But that won't work with auto. So presumably either of:

 if (int index; (index = a_list.indexOf(A("ONE"))) > -1)
 if (auto index = a_list.indexOf(A("ONE")); index > -1)

Christian Ehrlicher

If you want to use c++17 you should learn the syntax...

if (auto index = a_list.indexOf("ONE"); index > -1)

sbela

This works! But this is not what I want to write... :)
I want that "index" to be local to that "if" - if that is possible?! :)

@KroMignon said in Qt6.2.1 QList index out of bound:

int index;
if ((index = a_list.indexOf(A("ONE"))) > -1)
{
std::cout << "Item index:" << index << std::endl;
a_list.removeAt(index);
}

sbela

This works! Thank you!

if (int index = a_list.indexOf(A("ONE")); index > -1)

@Christian-Ehrlicher