Qt 5.5 - QList variable causes compilation to fail

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