filesystem in C++17 and Q_OBJECT conflicts

[jronald]

Env

Qt 6.2.1
Linux

Sample

CMakeLists.txt

...
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
...

C++ code

#include <filesystem>
#include <QApplication>

class MyApp : public QApplication
{
    Q_OBJECT
    ...
}

Error

usr/include/c++/11.1.0/bits/fs_fwd.:39:1: error: Parse error at "std"

More Info
If remove #include <filesystem>, it builds successfully.

---

EDIT
If #include <filesystem> after #include <QApplication>, the problem disappers, why?

#include <QApplication>
#include <filesystem>

[VRonin]

This is a bug in MOC: https://bugreports.qt.io/browse/QTBUG-73263 what they suggest is to reorder the includes as you already figured out

[JonB]

@jronald said in filesystem in C++17 and Q_OBJECT conflicts:

Parse error at "std"

Purely at a guess <filesystem> needs <std> stuff to be included first? <QApplication> includes that so makes it work if included before <filesystem>? Or something along those lines. Does not sound like that should be so, but it's what you describe as behaviour....

usr/include/c++/11.1.0/bits/fs_fwd.:39:1: error: Parse error at "std"

I am surprised that path does not start with /usr? I am no expert, but should a C++17 be including a c++/11.1.0 header file, is that a possible issue?

Actually your error is reported at https://stackoverflow.com/questions/69407237/qt-moc-errorusr-include-c-10-bits-fs-fwd-39-parse-error-at-std one month ago. Not sure what the "solution" is, but that's the issue....

[jronald]

@JonB
tried

#include <std>
#include <filesystem>
#include <QApplication>

But <std> doesn't exist.

[raven-worx]

@jronald
what compiler (version) are you using?

[JonB]

@jronald said in filesystem in C++17 and Q_OBJECT conflicts:

But <std> doesn't exist.

I didn't say it did, I was talking conceptually. In any case since you know

#include <QApplication>
#include <filesystem>

works, use that. Else you'll have to follow the discussion in that stackoverflow post.

[django.Reinhard]

Hi,

I use filesystem from stdlib without trouble. And without using QApplication in same file.

I suspect that errormessage from first post depends on files included before "filesystem" (not mentioned in your post).

The given line from error message looks like:

namespace std _GLIBCXX_VISIBILITY(default)

So may be 'namespace' has been declared as a macro or that GLib-Token does not exists. I can't imagine of other errors.

The include-file "std" (without extension) is a boost include file.

so may be, something is mixed without intention?

[jronald]

@raven-worx said in filesystem in C++17 and Q_OBJECT conflicts:

@jronald
what compiler (version) are you using?

g++ 11.1.0

[jronald]

@django-Reinhard said in filesystem in C++17 and Q_OBJECT conflicts:

Hi,

I use filesystem from stdlib without trouble. And without using QApplication in same file.

I suspect that errormessage from first post depends on files included before "filesystem" (not mentioned in your post).

The given line from error message looks like:

namespace std _GLIBCXX_VISIBILITY(default)

So may be 'namespace' has been declared as a macro or that GLib-Token does not exists. I can't imagine of other errors.

The include-file "std" (without extension) is a boost include file.

so may be, something is mixed without intention?

To reproduce the problem, two conditions must be meeted:

• #include <filesystem> before #include <QApplication>
• MyApp must inlcude Q_OBJECT

[JonB]

@jronald
You can keep asking questions and have others make guesses if you wish, but I have already given you the stackoverflow post which shows this error and indicates where the problem lies (i.e. in the toolchain).

As it states there, the error comes from moc. And that is only invoked if you have a Q_OBJECT macro present. It also tells you the problem lies in cmake (which is what you are using), but not in qmake. Hence your findings.

I'll leave you to it now.

[VRonin]

This is a bug in MOC: https://bugreports.qt.io/browse/QTBUG-73263 what they suggest is to reorder the includes as you already figured out

[britsfp]

@jronald
Hi, Here is something else: there is no <filesystem> file on my installation? How do I get this?

Regards
Frik Brits.

[jsulm]

@britsfp What compiler do you have?
You need C++17 capable compiler and also C++17 needs to be activated in pro file when using g++/MinGW (CONFIG += c++17).

[britsfp]

@jsulm I am using Qt 6.2.1. In the Kits's Compile tab It has gcc 7 and gcc 10 for C programs.
For C++ compiler it has g++ 7 and g++ 10

I could always change the soft link in /usr/bin ??

[jsulm]

@britsfp said in filesystem in C++17 and Q_OBJECT conflicts:

I could always change the soft link in /usr/bin ??

There is no need for such hacks.
Simply define the compiler in your Kit in QtCreator.

[britsfp]

@jsulm
Ok So must both C and C++ be version 10?, I supose so ? - and what is this #include <filesystem> error I get from the qfile.h header?

[jsulm]

@britsfp C doesn't matter most of the time, unless you want to compile C code.
But usually should point to same compiler version.

"and what is this #include <filesystem> error I get from the qfile.h header?" - as I said already: you need a compiler which supports C++17 standard and you need to enable this standard in your pro file.
https://en.cppreference.com/w/cpp/filesystem

[JoeCFD]

@britsfp g++ 7 does not support <filesystem>. If you can, set both C and C++ to 11 which supports C++20

[chavsur]

i have also faced same thing while compilation. .