I have a problem with adding an element to a QListWidget. I have build some frame with QtDesigner and then, I want to add some elements to a list in code. Even when I write:
QListWidgetItem* i = new QListWidgetItem("text");
Q_ASSERT(stepsList);
qDebug() << "before";
stepsList->addItem(i);
qDebug() << "after";
It prints only "before" and crashes with SIGSEGV. Additionaly, I managed to get such error message with this:
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff6f2a4a4 in QListWidget::count() const ()
from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
What is the reason?
Well, acceptation is needed so I'll write what was wrong:
I needed to call setupUi() first, in order to initialize the stepsList as #Timo Geusch wrote.
Solved.
Related
I've experienced a weird crash when trying to find a QLineEdit in a QFormLayout.
Firstly, I created a QFormLayout and set a name for it:
QFormLayout *charColLayout = new QFormLayout; charColLayout->setObjectName("charColLayout");
Then, I created a QLineEdit, modified it a bit and add it in to the layout, and I also give it a name:
QLineEdit *delim = new QLineEdit;
delim->setMaxLength(1);
delim->setMaximumWidth(100);
delim->setText("/");
delim->setObjectName("delEdit");
charColLayout->addRow("Delimiter", delim);
Afterward, in a completely different function, I re-searched that layout with findChild():
QFormLayout *charcoal = secondHoriField->findChild<QFormLayout *>("charColLayout", Qt::FindChildrenRecursively);
It should be noted that secondHoriField is just a normal QLayout which my QFormLayout is located in.
Finally, I attempted to find that QLineEdit:
QLineEdit *delimEdit = charcoal->findChild<QLineEdit*>("delEdit", Qt::FindChildrenRecursively);
if (delimEdit == nullptr) {cerr << "error\n";} //debug
string curDelim = qStrToStr(delimEdit->text());
And it surprisingly came down with a crash, and as the output shown, it's because the delimEdit is null.
18:06:10: Starting D:\...\build-cryptog-Desktop_Qt_5_15_2_MinGW_64_bit-Debug\debug\cryptog.exe ...
error
18:06:17: The program has unexpectedly finished.
18:06:17: The process was ended forcefully.
18:06:17: D:\...\build-cryptog-Desktop_Qt_5_15_2_MinGW_64_bit-Debug\debug\cryptog.exe crashed.
But when I switched the findChild() function for this rather bruteforce-y line:
QLineEdit *delimEdit = dynamic_cast<QLineEdit*>(charcoal->itemAt(1)->widget());
cerr << qStrToStr(delimEdit->objectName()) << endl; //debug line
The program ran fine, and it showed the same name I set for the QLineEdit:
18:12:02: Starting D:\...\build-cryptog-Desktop_Qt_5_15_2_MinGW_64_bit-Debug\debug\cryptog.exe ...
delEdit
18:12:11: D:\...\build-cryptog-Desktop_Qt_5_15_2_MinGW_64_bit-Debug\debug\cryptog.exe exited with code 0
Why did this happened?
Another note: qStrToStr() is a function I implement to convert QString to std::string, and I have hand-checked it.
While findChild is a QObject method the itemAt is a QFormLayout method.
QObject::findChild
QFormLayout::itemAt
With addRow you add an item to the QFormLayout. This does not make it a child in the context of the QObject.
QFormLayout::addRow
The purpose of the QFormLayout is to organize the positioning of QWidgets, it is not meant to serve as a container. Maybe you could check whether the top level QWidget (e.g. QMainWindow) holding the QFormLayout would be a better choice as a parent for the QLineEdit.
Assuming you have some kind of QMainWindow:
QMainWindow myMainWindow;
// ...
QLineEdit *delim = new QLineEdit(&myMainWindow);
delim->setObjectName("delEdit");
//...
In another location:
auto delimEdit = myMainWindow.findChild<QLineEdit*>("delEdit");
I have some code use qtcpsocket to write and read,
write-->sleep-->read;
and ui had 2 and more timer to use this function; by i want i run synchronous;so i add mutex to lock it; by it deadlock;
qt4; qt5;
void MainWindow::Start()
{
pTimer = new QTimer(this);
pTimer->setInterval(100);
connect(pTimer,SIGNAL(timeout()), this, SLOT(OnTimer()) );
pTimer->start();
pTimer2 = new QTimer(this);
pTimer2->setInterval(100);
connect(pTimer2,SIGNAL(timeout()), this, SLOT(OnTimer()) );
pTimer2->start();
}
void MainWindow::OnTimer()
{
FunCal(); // in my real code it will MyObj.DoSometing();
}
void MainWindow::FunCal()
{
qDebug()<<"log in fun...";
QMutexLocker loc(&this->_mtx);
qDebug()<<"getted lock in fun...";
QEventLoop loop;
QTimer::singleShot(100, &loop, SLOT(quit()));
loop.exec();
qDebug()<<"log out fun...";
}
I Want i run and out put as:
log in fun...
getted lock in fun ...
log out fun...
log in fun...
getted lock in fun...
log out fun...
but It Run like this:
log in fun...
getted lock in fun ...
log in fun....
---------------------------------no more ---------------------
IMHO the issue of OP results from a basic misunderstanding:
QTimer doesn't introduce multithreading.
It's just a facility to queue events which will sent after a certain time.
That's why, the QEventLoop is necessary to make it running at all.
However, it's still a deterministic execution and this is what probably happens inside the code of OP:
pTimer->start(); → starts first timer
pTimer2->start(); → starts second timer
control flow returns to event loop of QApplication (not exposed in code)
first timer becomes due and calls MainWindow::FunCal()
qDebug()<<"log in fun..."; → output of log in fun...
QMutexLocker loc(&this->_mtx); → this->_mtx becomes locked
qDebug()<<"getted lock in fun..."; → output of getted lock in fun...
loop.exec(); → enter a nested event loop (Nested event loops are allowed in Qt.)
second timer becomes due and calls MainWindow::FunCal() (Please, remember that it was immediately started after first with same interval time.)
qDebug()<<"log in fun..."; → output of log in fun...
QMutexLocker loc(&this->_mtx); → PROBLEM!
To illustrate it further, imagine the following call stack at this point (above called below):
QApplication::exec()
QEventLoop::exec()
QEventLoop::processEvents()
QTimer::timerEvent()
QTimer::timeOut()
MainWindow::onTimer()
MainWindow::FunCal()
QEventLoop::exec()
QTimer::timerEvent()
QTimer::timeOut()
MainWindow::onTimer()
MainWindow::FunCal()
QMutexLocker::QMutexLocker()
QMutex::lock()
(Note: In reality, you will see much more entries in the call-stack which I considered as irrelevant details in this case.)
The problem is: This second call of MainWindow::FunCal() cannot lock the mutex because it is already locked. Hence, the execution is suspended until the mutex is unlocked but this will never happen. The locking of mutex happened in the same thread (in the first/outer call of MainWindow::FunCal()). Unlocking would require to return from this point but it cannot because it's suspended due to the locked mutex.
If you think this sounds like a cat byting into its own tail – yes, this impression is right. However, the official term is Deadlock.
The usage of a QMutex doesn't make much sense as long as there are no competing threads. In a single thread, a simple bool variable would do as well because there are no concurrent accesses possible in a single thread.
Whatever OP tried to achieve in this code: Concerning the event-based programming forced/required by Qt, the problem is simply modeled wrong.
In single threading, a function cannot be entered twice accept by
a (direct or indirect) recursive call
a call out of a triggered interrupt handler.
Leaving the 2. aside (irrelevant for OPs Qt issue), the recursive call happens explicitly due to establishing a second (nested) event loop. Without this, the whole (mutex) locking is unnecessary and should be removed as well.
To understand event-based programming in general – it's described in the Qt doc. The Event System.
Additionally, I found Another Look at Events by Jasmin Blanchette which IMHO gives a nice little introduction into how the Qt event-based execution works.
Note:
Event-based programming can become confusing as soon as the amount of involved objects and signals becomes large enough. While debugging my Qt applications, I noticed from time to time recursions which I didn't expect.
A simple example: A value is changed and emits a signal. One of the slots updates a Qt widget which emits a signal about modification. One of the slots updates the value. Hence, the value is changed and emits a signal...
To break such infinite recursions, a std::lock_guard might be used with a simple DIY class Lock:
#include <iostream>
#include <mutex>
#include <functional>
#include <cassert>
// a lock class
class Lock {
private:
bool _lock;
public:
Lock(): _lock(false) { }
~Lock() = default;
Lock(const Lock&) = delete;
Lock& operator=(const Lock&) = delete;
operator bool() const { return _lock; }
void lock() { assert(!_lock); _lock = true; }
void unlock() { assert(_lock); _lock = false; }
};
A sample object with
a property-like member: bool _value
a simplified signal emitter: std::function<void()> sigValueSet
and a lock used to prevent recursive calls to setValue(): Lock _lockValue
// a sample data class with a property
class Object {
private:
bool _value; // a value
Lock _lockValue; // a lock to prevent recursion
public:
std::function<void()> sigValueSet;
public:
Object(): _value(false) { }
bool value() const { return _value; }
void setValue(bool value)
{
if (_lockValue) return;
std::lock_guard<Lock> lock(_lockValue);
// assign value
_value = value;
// emit signal
if (sigValueSet) sigValueSet();
}
};
Finally, some code to force the lock into action:
#define DEBUG(...) std::cout << #__VA_ARGS__ << ";\n"; __VA_ARGS__
int main()
{
DEBUG(Object obj);
std::cout << "obj.value(): " << obj.value() << '\n';
DEBUG(obj.sigValueSet = [&](){ obj.setValue(obj.value()); });
DEBUG(obj.setValue(true));
std::cout << "obj.value(): " << obj.value() << '\n';
}
To keep things short, I connected a slot to the signal which directly sets value again while the signal is emitted.
Output:
Object obj;
obj.value(): 0
obj.sigValueSet = [&](){ obj.setValue(obj.value()); };
obj.setValue(true);
obj.value(): 1
Live Demo on coliru
For a counter-example, I excluded the test if (_lockValue) return; and got the following output:
a.out: main.cpp:18: void Lock::lock(): Assertion `!_lock' failed.
Object obj;
obj.value(): 0
obj.sigValueSet = [&](){ obj.setValue(obj.value()); };
obj.setValue(true);
bash: line 7: 12214 Aborted (core dumped) ./a.out
Live Demo on coliru
This is similar to what happened in OPs case with the only difference that in my case double-locking just violated the assert().
To make this complete, I exluded the lock guard std::lock_guard<Lock> lock(_lockValue); as well and got the following output:
execution expired
Live Demo on coliru
The execution was trapped into an infinite recursion, and the online compiler aborted this after a certain time. (Sorry, coliru. I won't do it again.)
I didn't find a proper solution to this problem, so I hope somebody can give me an answer to my problem:
I am using a normal QTreeWidget, but as items I use an own subclass of QTreeWidgetItem (because I needed to store some other information in the item). Now I want to use the itemClicked() signal by the QTreeWidget, but I think my slot doesn't get any signal and I think it has to do with the signature of itemClicked(), since it sends a QTreeWidgetItem and of course not my own subclass.
Is it possible that QTreeWidget doesn't detect a click on my own subclass items?
Here is my connection:
connect(treeWidget, SIGNAL(itemClicked(QTreeWidgetItem *)), this, SLOT(displayTransformData(QTreeWidgetItem*)));
And here is my slot:
void GUI::displayTransformData(QTreeWidgetItem* item) {
cout << "test" endl;
Q_actorTreeWidgetItem* actor_item = dynamic_cast<Q_actorTreeWidgetItem*>(item);
vtkSmartPointer<vtkActor> actor =
vtkSmartPointer<vtkActor>::New();
actor = actor_item->getActorReference();
double* position = new double[3];
position = actor->GetOrigin();
x_loc->setText(QString(std::to_string( position[0]).c_str() ));
}
I'm already trying to cast the item that I could get from the signal into my own subclass, but the slot function is never called, because the test from my cout line doesn't appear in the console.
I'm very grateful for every help!
The problem is your incorrect SIGNAL specification,
SIGNAL(itemClicked(QTreeWidgetItem *))
You should probably see a warning message at the console along the lines of:
QObject::connect: No such signal
tree_widget::itemClicked(QTreeWidgetItem *) in ...
From the documentation the actual signal spec is
void QTreeWidget::itemClicked(QTreeWidgetItem *item, int column)
So, using the old Qt4 syntax you need
connect(treeWidget, SIGNAL(itemClicked(QTreeWidgetItem *, int)),
this, SLOT(displayTransformData(QTreeWidgetItem*)));
If possible, however, you should make use of the newer Qt5 signal/slot syntax
connect(treeWidget, &QTreeWidget::itemClicked, this, &GUI::displayTransformData);
I use QProcess and connect it's readyReadStandardOutput to slot. But after starting slot execute twice. Tell me please why is it?
{
myProcess = new QProcess(parent);
myProcess->start("mayabatch.exe -file "+scene);
connect(myProcess, SIGNAL(readyReadStandardOutput()), this, SLOT(readOutput()));
}
void MainWindow::readOutput()
{
qDebug()<<"Read";
QByteArray outData = myProcess->readAllStandardOutput();
qDebug()<<QString(outData);
}
OUTPUT:
Read
"File read in 0 seconds.
"
Read
"cacheFi"
Read
"le -attachFile -fileName "nClothShape1" -directory ...
Last string was broken. "Read" appears between words.
From the documentation of QProcess::readyReadStandardOutput()
This signal is emitted when the process has made new data available through its standard output channel (stdout). It is emitted regardless of the current read channel.
The slot is executed more than once for the simple reason that the underlying process flushed the output in separate and random ways. You should not be caring about this because it depends on things you cannot control.
If you want to save the whole output you should be doing
void MainWindow::readOutput(){
bigbuffer.append(myProcess->readAllStandardOutput();)
}
If you want to read line by line, then
void MainWindow::readOutput(){
while(myProcess.canReadLine()){
qDebug() << myProcess.readLine();
}
}
The second call will leave data in the process buffer such that you don't have "broken" reads like cacheFi.
I am trying to get touch inputs for my program targeting an N8 (and a C7), and I am not able to create a QList for keeping touchpoints using QTouchEvent::touchPoints(). The program crashes with the following line: Thread has crashed: A data abort exception has occurred accessing 0xee
The overloaded events function looks like:
bool GLWindow::event(QEvent *event)
{
switch ( event->type() ) {
case QEvent::TouchBegin: {
QList<QTouchEvent::TouchPoint> touchBeginPoints =
static_cast<QTouchEvent *>(event)->touchPoints();
foreach (const QTouchEvent::TouchPoint &touchBeginPoint, touchBeginPoints)
{
float touchBeginX = touchBeginPoint.pos().x();
float touchBeginY = touchBeginPoint.pos().y();
qDebug() << "touchBeginPoint := " << touchBeginX << ", " << touchBeginY;
}
break;
}
case QEvent::TouchUpdate: {
// same as touch begin: getting touch point
break;
}
case QEvent::TouchEnd: {
// same as touch begin: getting touch point
break;
}
default: {
qDebug() << "Goodbye";
return true;
}
}
return true;
}
Now,
I have never worked with containers before. But creating and using a QList in another part of the program works fine. Should I be including something in my .pro file? (Most problems seem to end up regarding this with me!)
I read (a bit) about exceptions in Qt and Symbian, but I am not able to get most of that. BUT I am not doing any networking or resource based i/o or manipulation except textures for 3D objects. Is it possible that memory allocation while running the program is creating some problem?
Basically I am just trying to print the touch point. But I am clueless as to why I can’t create a QList. The code compiles fine. I tried my best (unsuccessfully), but is there any other way to get the screen coordinates of a touchpoint (one that does not require a QList)? Any comments are welcome.
[Reposting from qt-project.org.]
Your syntax is 100% correct. Just look at this example: http://www.developer.nokia.com/Community/Wiki/Painting_in_Qt
What I'm guessing happens is that QTouchEvent::touchPoints() returns a list big enough that it overflows your stack. Try increasing the stack size for your application.
Is your syntax correct ? The compilation error seems to reinforce teukkam point...
What happens when you replace
static_cast<QTouchEvent *>(event)->touchPoints()
With
(dynamic_cast<QTouchEvent *>(event))->touchPoints()
Notice the parentheses...