QT missing dll after deploy - c++

Question

I've copied all of the dlls from QT that were required, and my application works fine on my Windows server machine.
However when trying to run it on a Windows 7 box i get the following message:
This application failed to start because it could not find or load he
Qt platform plugin "windows".
Reinstallning the application may fix this problem.
Any ideas what I'm missing here?

Answer 1

I'd scratched my head over this some time ago. It turned out that this was caused not by missing qwindows.dll, but rather one of libEGL.dll or libGLESv2.dll. This was tricky, because dependency walker does not show those libs as direct dependencies.
If you want to test on your dev machine, whether your app has all required libs, fire up console issue SET PATH=, cd to your app directory and run it.
This is complete list of dlls that my app is using (Qt 5.2 / QtQuick app only, rest is C++). QtQuick is nice but the size of Qt dependencies is a bit scary:
icu*.dll - depending on whether you've compiled with ICU
libEGL.dll
libGLESv2.dll
Qt5Core.dll
Qt5Gui.dll
Qt5Network.dll
Qt5Qml.dll
Qt5Quick.dll
Qt5Widgets.dll

Answer 2

Widely used solution is put all necessary libraris in the folder of application.
What are libraries application need?
Run application and see error message:
The program can't start because <Library name> is missing from your computer.
Try reinstalling the program to fix this problem
Library set is depended from Qt version. Run several times application and each time copy required lib you found what is neeeded for application.
In my case (Qt 5.2.1) there are
icudt51.dll,
icuin51.dll,
icuuc51.dll,
libgcc_s_dw2-1.dll,
libstdc++-6.dll,
libwinpthread-1.dll,
Qt5Core.dll,
Qt5Gui.dll,
Qt5Widgets.dll.
All libs you can found in your Qt install folder. But don't use libraries from Tools\QtCreator folder, because QtCreator has another version of these libraries!
In case of error:
This application failed to start because it could not find or load he Qt platform plugin windows. Reinstallning the application may fix this problem.
You should create folder platforms and copy qwindows.dll into it.
If you still got error you should create qt.conf file in application's folder with content:
[Paths]
Plugins=plugins
This solution is described in https://qt-project.org/forums/viewthread/37265
More information about qt.conf you can find at http://qt-project.org/doc/qt-5/qt-conf.html
In latest versions of Qt you can find deploy tool (since 5.2). This tool find necessary libraries for application and copy into application folder. You can run it something like this:
call c:\Qt\QtX.Y.Z\X.Y.Z\mingw48_32\bin\qtenv2.bat
cd /d "c:\path\to\your\application\folder"
windeployqt.exe your_application.exe
Generally it works well. But I notice that some libraries are not copied, but you can found by method is descibed at beggining of post. More useful information you can find at
http://qt-project.org/doc/qt-5/windows-deployment.html
In rare cases yo can got this error if some library is missing but not appear in error message above. Example: Qt 5.1.1: Application failed to start because platform plugin "windows" is missing
I wasn't in this situation, so I can't tell more.