<Solved> Qt 5 and Qt 4 compatibility (#include <QtWidgets>) (reprise)

norisezp

On December 27, 2012 (yes, 2 years and more ago) DerekDomino's post was answered by Lukas Geyer by saying the folowing:

" #include <QtGui>
" #if QT_VERSION >= 0x050000
" #include <QtWidgets>
" #endif
"
"The recommendation however is to not use the module prefix anymore (#include <QApplication> "instead of #include <QtWidgets/QApplication>). It was never recommended to use module-wide "includes (#include <QtWidgets>)."

I have two questions about this for this august assemblage of minds:

Why does it work at all?
Why does the code in the examples use module-wide includes?

Thank you.

JKSH

Hi, and welcome to the Qt Dev Net!

Why? Simply because it was designed to work both ways in the beginning. However, people who wrote <QApplication> did not face issues when porting from Qt 4 to Qt 5, compared to people who wrote <QtGui/QApplication> or even <QtGui>.

For simplicity, I guess. I use module-wide includes to quickly try something new. When my project becomes more mature, I replace the module-wide includes with individual-class includes. This reduces compilation times.

norisezp

I see. Thank you, JKSH.

I think this is still on topic. Can anyone explain why this does not work on 5.4.0 Windows MinGW? It doesn't seem to be defining all the Qt classes:

#if QT_VERSION >= 0x050000
#include <QtWidgets>
#else
#include <QtGui>
#endif

Compiles fine on 4.8 Ubuntu, but receives errors indicative of missing class definitions on Windows. If I remove the conditional compile directives and simply use #include <QtWidgets> work fine on Windows. However, I don't find this conducive to maintaining a portable code base. Thanks.

norisezp

Darn it, the editor widget played with my posting! The code snippet should have read (I hope this goes through all right:

@
#if QT_VERSION >= 0×050000
#include <QtWidgets>
#else
#include <QtGui>
#endif
@

[EDIT: Added '@' around code snippets -- JKSH]

JKSH

I'm not sure. Have you double-checked that QT_VERSION >= 0×050000 is indeed returning true?

What are the exact error messages you saw?

Also, what's your rationale for supporting both Qt 4 and Qt 5?

andreyc

Also in Qt5 you need to add
@
QT += widgets
@
to .pro file if you use qmake as a build tool.

norisezp

@andreyc - I'm using "QT += core gui widgets" which QtCreator finds acceptable, and it compiles OK if I don't use the conditional in the source file.

@JKSH - error: invalid use of incomplete type 'class QWidget'
class BlahBlah : public QWidget
^
I'm also deploying on Ubuntu, and Canonical only supports 4.8. I really don't want to blaze new trails, so I use what they supply.

norisezp

Oh, and the #if is working. My evidence is that a #error inserted below the #include <QtWidgets> gets executed.

norisezp

Well, here is the answer that I found:

@#include <qglobal.h>
#if QT_VERSION >= 0×050000
#include <QtWidgets>
#else
#include <QtGui>
#endif@