Application failed to start because it could not find or load the QT platform plugin "windows"

The error is caused because the program can't find qwindows.dll

qwindows.dll has to be in a folder named platforms so that the path from your executable to the dll is platforms/qwindows.dll

Whereas this wasn't enough in my case. I had also to add following line at the beginning of my main()

QCoreApplication::addLibraryPath("./");

Then everything worked.

The application is able to run on the host system, since the Qt bin path is in the system PATH variable.

There is a standard Qt tool for deployment of Qt applications on Windows windeployqt to be able to run the application on target machines that do not have Qt installed.

That tool takes care about Qt DLL dependencies, makes a copy of platforms\qwindows.dll and also it makes a copy of libraries that you cannot detect with the Dependency Walker, since image plugins and some other DLLs are loaded at runtime.

You do not even need to have your Qt bin folder in your environment PATH. The simplest deployment:

• copy built exe binary to a new folder
• open cmd console in that folder
• call windeployqt using the full path (if it is not in the system PATH) and provide your executable, for example:

  c:\Qt\Qt5.2.1\5.2.1\msvc2010_opengl\bin\windeployqt.exe application.exe
  

As a result you have in that folder all needed Qt DLLs to run the application.

The tool windeployqt has various options. It can also take care about deployment of qml related files.

Of course you can have also issues with MSVC redistributables, but those should be deployed separately and installed once per system.

Only some 3rd party libraries should be copied manually if they are used, for example OpenSSL.

I got this issue and how I solved it:

1. Used dependency walker(http://www.dependencywalker.com/) to see the exact path of the dlls needed. Try it because both QtCreator and QT framework both have the same dlls and you must pinpoint the exact ones used. I copied all dlls needed in the same folder as the app.

2. I have copied the folder platforms from QT framework /plugins and copied it in the same folder as the app. Now the app comtained also plugin/platform/ folder with all its dlls

3. And the most important step in my case is to create a file named qt.conf in the same folder as the app . This file should contain the path to the plugins. My qt.conf file contains:

   [Paths]
   Libraries=../lib/qtcreator
   Plugins=plugins
   Imports=imports
   Qml2Imports=qml