Cannot work out why this segfaults

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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);
@

jeremy_k

[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 *>

JKSH

[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.

jeremy_k

[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

? Offline

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!

JKSH

[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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I can show you the proper code, I notice you are on Freenode. I tried to whois you but apparently you are not online.

JKSH

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.

jeremy_k

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