Qt 5.5 - QList variable causes compilation to fail

dheerendra

Your error may be coming line previous to this line. Just check. I don't see any issue with above line of code.

knight556

I wish it was that easy, but if that line is commented out, the rest of the program runs perfectly.

mrjj

@knight556 said:

And the
#include <QList>
#include <QString>

are in the .H file ?

with the class having the m_mylist?

Are you using Creator and a pro file or how do you build the project ?

knight556

The includes are in the .cpp file, and yes I'm using Creator with a pro file. I've noticed that if I declare a QList within a class method then it compiles just fine, it's only when I try and declare it as a member variable that it complains.

SGaist

Hi and welcome to devnet,

Not a direct answer but since you want a QList of QString, why not use QStringList ?

mrjj

@knight556
Hi

But if you declare m_list in the H file you must have
#include <QList>
#include <QString>

in the H file (and not cpp file ) or the type is unknown :)
Its not enough to include in CPP file.

I have noticed that if I declare a QList within a class method ...

Yes as you included the files in the cpp I guess.

knight556

This post is deleted!

knight556

@mrjj Moving the includes to the .h file didn't help.

@SGaist Yes the QStringList does in fact work, but what I have failed to mention (for simplicity) is that this bug is occurring for all data types, eg. QList<int> fails as well. As it turns out I want to use my own custom class as the list type, but I can't hope to get that working until I get the QList working with more basic data types first!

Creator is clearly happy with QList being used, because if I start typing on the next line something like:
m_mylist.
the autocomplete/intellisense/whatever-it's-called will give me the option of append(), so something is working correctly!

mrjj

@knight556

Oh ok. sorry for assuming it was that basic thing,

Can we try a simple test just to be sure compiler not funky?

If you create a new console project and then
right click -> add new -> Header file and call it thehappylist.h
and put code below in thehappylist.h

#ifndef THEHAPPYLIST
#define THEHAPPYLIST

#include <QList>
#include <QString>
class Happy {
public:
    QList<QString> m_list;
};
#endif // THEHAPPYLIST

and then the main.cpp

#include "thehappylist.h"
int main(int argc, char *argv[])
{
    Happy h;
    h.m_list.append("test");
}

And tell me if that compiles ?

knight556

@mrjj said:

#include "thehappylist.h"
int main(int argc, char *argv[])
{
Happy h;
h.m_list.append("test");
}

Hi, yes it does compile... which is frustrating because that doesn't help find the problem.

mrjj

HI

Well, its not an include path thing then.
Since the mini sample works.

Would it be possible to post your code ?
I really can not get a clue of what could be wrong as you have the includes
and it all sounds ok.

knight556

I'm working on quite a large project (it's basically a DICOM image viewer), but I will attempt to create a minimal working example.

knight556

OK now this is weird. I'm not sure how I came to this, but instead of writing:

QList<QString> m_mylist;

I write:

::QList<QString> m_mylist

It compiles and works... however Qt Creator doesn't seem to like it as the terms QList and QString are not coloured purple as normal.
Any ideas why this works?

JKSH

@knight556 said:

OK now this is weird. I'm not sure how I came to this, but instead of writing:

QList<QString> m_mylist;

I write:

::QList<QString> m_mylist

It compiles and works... however Qt Creator doesn't seem to like it as the terms QList and QString are not coloured purple as normal.
Any ideas why this works?

Did you redefine "QList" somewhere else in your code?
Does this only happen in this particular class, or does it also happen if you add a QList member to other classses?

knight556

Well that got me thinking, so I had a look and for some reason in another one of my classes I had listed a forward reference to QList. I have now deleted that line of code and the rest of the program compiles and works as expected, without needing the extra :: bits at the start of the QList declaration!
So... clearly I don't understand forward references. Thanks for the help everyone!

SGaist

It's pretty unusual to forward declare QList, why did you do that in the first place ?

knight556

I was learning Qt through a series of Pluralsight videos and the instructor was basically forward declaring every class he was gonna use, so I got into that habit. Time to go back and remove some of that rubbish.

SGaist

Even QString ?

knight556

Yep even QString. Is forward referencing even necessary if you put all your includes in the header file? I always thought forward referencing was only needed if you had 2 classes that referenced one another, but of course 1 had to be defined before the other.

JKSH

@knight556 said:

Is forward referencing even necessary if you put all your includes in the header file?

No. The header contains more information than the forward declaration, so it doesn't make sense to forward-declare when you already have the header.

I always thought forward referencing was only needed if you had 2 classes that referenced one another, but of course 1 had to be defined before the other.

That's only one use of forward declarations.

It is good practice to minimize the amount of code that you #include inside your headers. When your project gets large, forward declarations really help to speed up compilation times. See http://stackoverflow.com/questions/4757565/c-forward-declaration