Cannot work out why this segfaults
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Hi Jeremy,
I have updated my post so hopefully everything looks better now.
It is actually this line: @ qDebug() << "The value of the test string is " + this->test->returnTestString();@ that is causing the crash. It is when I call the method I get a crash.
I have check two things:
- this->test is not NULL. I can get an address in memory with &this->test.
- It is not the concatenation of strings that is causing this error. Just calling this->test->returnTestString(); on a separate line causes a crash.
I have sent you a message.
Thanks
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Hi, and welcome to the Qt Dev Net!
[quote]this->test is not NULL. I can get an address in memory with &this->test.[/quote]This alone isn't enough to prove that your pointer is OK. Uninitialized or dangling pointers can be non-NULL too.
(If you haven't assigned anything to the pointer, it's uninitialized. If you assign an object to a pointer but then destroy the object, the pointer is dangling. These concepts apply to C pointers too.)
How did you create your TestClass object?
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I am worried the garbage collector is destroying the object or something.
TestClass is created in a QList.
@function QList<TestClass> *buildTests()
{
QList<TestClass> *tests = new QList<TestClass>();
TestClass test = new TestClass("Hello World");
tests->append(*test);
return tests;
}@I have a class later that takes this QList, iterates in a drop down menu. Later when the user activates the button clicked slot:
@
TestClass *testreturn;
for (int i = 0; i < testList->count(); i++)
{
TestClass test= devicesList->at(i);
if (test.returnTestString() == ui->testSelectionBox->currentText())
{
testreturn = &test;
}
// Later on:
emit(test);
@ -
[quote author="sn1994" date="1409651189"]I am worried the garbage collector is destroying the object or something.
[/quote]Stop worrying! There is no garbage collector, unless you've added one or are using another framework that comes with one.
I see that the QList contains TestClass objects, rather than pointers to the objects. Does TestClass implement a copy constructor and assignment operator? If not, I wonder if the default implementation is the problem.
http://www.cplusplus.com/articles/y8hv0pDG/ describes what these are and why you might need to define customized versions.
Alternatively, change the QList<TestClass> to QList<TestClass *>
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[quote author="sn1994" date="1409651189"]
@
TestClass test= devicesList->at(i);
//...
testreturn = &test;
@
[/quote]
test is a local variable. testreturn is a pointer that points to that local variable.test will be destroyed when it goes out of scope.
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[quote author="JKSH" date="1409655317"][quote author="sn1994" date="1409651189"]
@
TestClass test= devicesList->at(i);
//...
testreturn = &test;
@
[/quote]
test is a local variable. testreturn is a pointer that points to that local variable.test will be destroyed when it goes out of scope.[/quote]
Good point. There's also the emit(test) which may not be what is intended.
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Sorry, I am doing emit (testreturn). I had to make my code clean to post here, so that is just a mistake in posting.
Should I make testreturn a public: variable and use memcpy to copy it? Oddly the first slot is OK with receiving the TestClass*, it is only later when I try and reference the global version I assign with this.test it crashes
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[quote author="sn1994" date="1409657238"]Sorry, I am doing emit (testreturn).[/quote]That doesn't work either: You need to emit a signal. What is the name of your signal?
[quote]Should I make testreturn a public: variable and use memcpy to copy it?[/quote]No, avoid using public variables. Also avoid using low-level functions like memcpy and malloc -- use the C++ equivalents instead (e.g. std::copy, new)
[quote author="sn1994" date="1409657238"]Oddly the first slot is OK with receiving the TestClass*, it is only later when I try and reference the global version I assign with this.test it crashes[/quote]It's still unclear to me what exactly your code is doing. Can you please post a minimal but complete example? Strip out all the parts that don't contribute to the segfault, and then post the full code for main(), MainWindow, and TestClass.
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You can post it here :) I haven't been on Freenode for a long time; I should probably update my profile.
[quote author="sn1994" date="1409657846"]p.s. I see your suggestion JKSH
At the moment I have QList<TestClass> testlist*, do you mean change it to
QList<TestClass*> testlist*
or QList<TestClass*> testlist
Thanks![/quote]The suggestion actually came from jeremy_k. I believe he meant QList<TestClass*> testlist
You almost never need to allocate a QList itself using new. It is an implicitly shared class, so we just copy it around or pass it by const-reference. See http://qt-project.org/doc/qt-5/implicit-sharing.html for details.
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Yes, please. Post here. This allows future readers to benefit from the conversation. It also puts a little pressure on posters to write brief, on point samples.
Confirmation on QList<TestClass *> testList.
Having a list of pointers makes it clear that fetching an item, manipulating it, and then going out of scope:
- will not create a new object,
- will modify the original in the list
- will not cause the object (original or copy of) to be destroyed
