geting icon by name is not working - qt

I can't get icon lookup by name to work correctly. I copied the code from Gallery examples and repeated the same configuration in my project, but it doesn't work.
This is what I did:
1.Copy icons/gallery into the directory of my own project, this is the list of files:
icons/default
icons/default/20x20#3
icons/default/20x20#3/back.png
icons/default/20x20#3/menu.png
icons/default/20x20#3/drawer.png
icons/default/20x20#4
icons/default/20x20#4/back.png
icons/default/20x20#4/menu.png
icons/default/20x20#4/drawer.png
icons/default/20x20
icons/default/20x20/back.png
icons/default/20x20/menu.png
icons/default/20x20/drawer.png
icons/default/index.theme
icons/default/20x20#2
icons/default/20x20#2/back.png
icons/default/20x20#2/menu.png
icons/default/20x20#2/drawer.png
2.Added index.theme file into theme directory:
[Icon Theme]
Name=default
Comment=Qt Quick Controls 2 Gallery Example Icon Theme
Directories=20x20,20x20#2,20x20#3,20x20#4
[20x20]
Size=20
Type=Fixed
[20x20#2]
Size=20
Scale=2
Type=Fixed
[20x20#3]
Size=20
Scale=3
Type=Fixed
[20x20#4]
Size=20
Scale=4
Type=Fixed
3.Added corresponding lines to main.cpp, to enable icons, this is the code:
QGuiApplication::setApplicationName("MyApp");
QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QIcon::setThemeName("default");
QQuickStyle::setStyle("Material");
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
In a QML file I use the icon this way:
ToolButton {
icon.name: "menu"
}
If I lookup by URL, it works fine:
ToolButton {
icon.source: "qrc:/icons/default/20x20/menu.png"
}
So, what could be the reason of why lookup by name doesn't work and how to debug this ?
EDIT:
After Mitch's answer I found that icon's weren't working because the ":/icons" entry in the icon's path wasn't the first one.
So, this code DOESN'T work:
QIcon::setThemeName("default");
QStringList list;
list<<":/icons";
list<<"/usr/share/icons";
QIcon::setThemeSearchPaths(list);
However, this cdoe DOES work:
QIcon::setThemeName("default");
QStringList list;
list<<":/icons";
list<<"/usr/share/icons";
QIcon::setThemeSearchPaths(list);
To make it work you only need to make the path where your icons are located, first entry in list.
But maybe it is a bug, I am using Qt 5.11

If the list of things you did is exhaustive, then you missed a step:
Traditionally, only Linux and UNIX support icon themes on the platform level, but it is possible to bundle a compliant icon theme in an application to use themed icons on any platform.
The default icon theme search paths depend on the platform. On Linux and UNIX, the search path will use the XDG_DATA_DIRS environment variable if available. All platforms have the resource directory :/icons as a fallback. Custom icon theme search paths can be set with QIcon::setThemeSearchPaths().
The following example bundles an icon theme called mytheme into the application's resources using Qt's resource system.
<RCC>
<qresource prefix="/">
<file>icons/mytheme/index.theme</file>
<file>icons/mytheme/32x32/myicon.png</file>
<file>icons/mytheme/32x32#2/myicon.png</file>
</qresource>
</RCC>
After creating that file, you also need to add it to your .pro.

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In other words, if my code for saving a file is as follows, I would like to know what to do in line 5 to make it work:
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Check this if you want to assign a custom icon file.
If you want to generate different preview for each file, just like image file, then check this.
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my qrc file defined as follows,
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....
I want to use the file background.png as my label's background.
I did like this,
label->setStyleSheet( "background-image: url(:/images/background.png);" );
but it cannot set the image as background.
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thanks
You said you want the /images prefix for the Resources/images/background.png file, so in the resource system the file is available as
:/images/Resources/images/background.png
If you want also to simplify the file's path, use an alias:
<file alias="background.png">Resources/images/background.png</file>
This will make it available under
:/images/background.png
For the future, learn how to debug such simple problems yourself: just putting
QDirIterator i(":/", QDirIterator::Subdirectories);
while (i.hasNext())
qDebug() << i.next();
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im developing QT application, and im using few icons
this is my resource.qrc file
<RCC>
<qresource prefix="/new/prefix1">
<file>army-officer-icon.png</file>
<file>uac.png</file>
</qresource>
</RCC>
then i ofcourse include it in my .pro file
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and here is the code which takes care of icons:
//this code is part of the mainwindow.cpp
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QLabel *label_5;
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===windows 7, developing station, QT installed
also the result effect is as i expected
i can see both icons :: tray icon & uac shield = awesome
but when i move to the other workstation, somethings strange happens
===windows XP, user work station, QT NOT installed
as u can see, the trayicon still has its own icon, but the "uac shield icon" dissapears... its very disturbing, and i really dont get it since both icons are *.png formats, i browsed a lot, and maybe it has connection with qt image plugins (althought i think that i should care about it only when my icon are *.jpeg, *.gif format), but wasnt able to make this solution works...
so any ideas are welcomed.
Thanx in advance.
Okay, ive found the solution just after ive posted this (actually , i was quite close, but i copied something with wrong path :/)
locate C:\QtSDK\Desktop\Qt\4.8.0\mingw\plugins
copy plugins/imageformats to your application folder
open main.cpp and add this line of codes
a.addLibraryPath(QCoreApplication::applicationDirPath ()+"/plugins");
compile, and have a look at your beautiful icons :)

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