Pass a method to QtConcurrent::map

nulluse

I thought so much! At some point I need to get my nose off the ground and look at the assembly listings of my code to really understand behind the scenes. That helped me a lot years ago when I was learning C++ using Stroustrup's 1st book and Borland C++ 2.0 compiler. Unfortunately I ended up doing almost nothing but Delphi and C# ever since and C++ was well forgotten by now.

Your help and patience with my silly questions is certainly appreciated!

kshegunov

@nulluse said:

Your help and patience with my silly questions is certainly appreciated!

I certainly wouldn't call them silly, this is pretty specific matter and as I said, in practice it mostly doesn't come up.

nulluse

When I was using Netbeans with GCC under Linux and FreeBSD, the below lambda declared in a descendant of QDialog worked fine:

auto func = [ // 96
	x0 = widget.spX0->value(), // 97
	y0 = widget.spY0->value(),
] (int& iteration) -> int { // 102
	return iteration;
};

widget.spX0 etc are the double spin boxes on the dialog.

But now I tried Qt Creator under Win7 and this lambda is throwing a bunch of errors:

newform.cpp:97: error: C2143: syntax error : missing ']' before '='
newform.cpp:97: error: C3481: 'x0': lambda capture variable not found
newform.cpp:102: error: C2059: syntax error : ']'
newform.cpp:97: error: C2059: syntax error : '='

Do I need to switch the language version to C++11 anywhere?

kshegunov

@nulluse said:

Do I need to switch the language version to C++11 anywhere?

Depending on the compiler and Qt version, probably, yes. You can add CONFIG += c++11 to your project file and if the compiler supports it it'll be enabled.

nulluse

That did not help.
This is what I just downloaded and installed with Qt 5.6 as the only version checked off in the setup:

Qt Creator 3.6.1
Based on Qt 5.6.0 (MSVC 2013, 32 bit)
Built on Mar 14 2016 09:57:09
From revision d502727b2c
Copyright 2008-2016 The Qt Company Ltd. All rights reserved.
The program is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

It has Desktop Qt 5.6.0 MSVC2013 64bit (default) under the Build&Run/Kits
Compiler is set to Microsoft Visual C++ Compiler 12.0 (amd64).

kshegunov

@nulluse
Hm. It should've worked. As far as I remember VS 2013 should support most of the C++11 features. Could you post the full compile line for the file you're getting the error at?

PS.
Also try declaring the variables for the capture explicitly and see how it goes:

int x0 = widget.spX0->value(), y0 = widget.spY0->value();
auto func = [ x0, y0] (int& iteration) -> int { // 102
    return iteration;
};

nulluse

14:41:29: Running steps for project MandelbrotQt...
14:41:29: Configuration unchanged, skipping qmake step.
14:41:29: Starting: "C:\Qt\Tools\QtCreator\bin\jom.exe" 
	C:\Qt\Tools\QtCreator\bin\jom.exe -f Makefile.Debug
	cl -c -nologo -Zc:wchar_t -FS -Zi -MDd -GR -W3 -w34100 -w34189 -w44996 -EHsc /Fddebug\MandelbrotQt.pdb -DUNICODE -DWIN32 -DWIN64 -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I..\..\MandelbrotQt -I. -IC:\Qt\5.6\msvc2013_64\include -IC:\Qt\5.6\msvc2013_64\include\QtWidgets -IC:\Qt\5.6\msvc2013_64\include\QtGui -IC:\Qt\5.6\msvc2013_64\include\QtANGLE -IC:\Qt\5.6\msvc2013_64\include\QtCore -Idebug -I. -IC:\Qt\5.6\msvc2013_64\mkspecs\win32-msvc2013 -Fodebug\ @C:\Users\user0\AppData\Local\Temp\newform.obj.11028.16.jom
newform.cpp
..\..\MandelbrotQt\newform.cpp(97) : error C2143: syntax error : missing ']' before '='
..\..\MandelbrotQt\newform.cpp(97) : error C3481: 'x0': lambda capture variable not found
..\..\MandelbrotQt\newform.cpp(102) : error C2059: syntax error : ']'
..\..\MandelbrotQt\newform.cpp(97) : error C2059: syntax error : '='
..\..\MandelbrotQt\newform.cpp(102) : error C2143: syntax error : missing ';' before '{'
..\..\MandelbrotQt\newform.cpp(105) : error C2065: 'x0' : undeclared identifier

Interesting enough, I started re-factoring the project to move the computations out of the GUI class, and my new class compiles fine when all I capture is [this], then refer to the member variables inside the lambda's body.

PS: Your workaround works. I will wrap that with #if defined (Q_OS_WIN) etc.

PPS: But now I am getting lots of linker errors similar to this:

newform.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl QtConcurrent::ThreadEngineBase::ThreadEngineBase(void)" (__imp_??0ThreadEngineBase@QtConcurrent@@QEAA@XZ) referenced in function "public: __cdecl QtConcurrent::IterateKernel<int *,void>::IterateKernel<int *,void>(int *,int *)" (??0?$IterateKernel@PEAHX@QtConcurrent@@QEAA@PEAH0@Z)

Do I need to add linking against a library when working under Windows?

PPPS: This does not make any sense: I am editing the .pro file adding and removing concurrent from QT but still getting this line in the output every time:

15:01:05: Configuration unchanged, skipping qmake step.

kshegunov

@nulluse said:

But now I am getting lots of linker errors similar to this:
Do I need to add linking against a library when working under Windows?

Only to the Qt modules as far as I know. Qt += core gui widgets concurrent etc. It should be working out of the box.

nulluse

See my last update - adding QT += concurrent does not change anything as the project thinks configuration is not changing.

kshegunov

@nulluse
Sometimes the IDE is not as smart and it doesn't understand the .pro has changed. Run qmake explicitly (somewhere in the build menu) and do a rebuild after that.

nulluse

Forcing rebuild does not change anything. The pro file looks like this now:

#-------------------------------------------------
#
# Project created by QtCreator 2016-04-18T13:54:41
#
#-------------------------------------------------

QT += core gui concurrent
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = MandelbrotQt
TEMPLATE = app
SOURCES += main.cpp\
        newform.cpp \
    helpers/fileHelper.cpp \
    mandelbrot/mandelbrot.cpp
HEADERS  += newform.h \
    helpers/fileHelper.h \
    mandelbrot/mandelbrot.h \
    ui_newForm.h
FORMS    += newform.ui
LIBS += -lQt5Concurrent
CONFIG += c++11

The linker is still complaining.

kshegunov

@nulluse
It looks okay to me, with the only exception of this line: LIBS += -lQt5Concurrent. You shouldn't need to pass that to the linker, adding the QT += concurrent is enough (qmake will generate the appropriate include files and linker switches).

nulluse

Running qmake fixed it!