Given the path "c:\Projects\C++\this is a rather long path for a path on windows where long paths are a problem with the limit of two hundred fifty characters but there is a solution which implies adding a registry key and configuring the project to include a manifest\object.h" with the content:

#include <QObject>

class A : QObject
{
    Q_OBJECT
public:
};

The following command works:

$ moc object.h > object.moc.cpp

I get a 2.67kB object.moc.cpp file. But the following command doesn't work:

$ moc object.h -o object.moc.cpp
moc: Cannot create object.moc.cpp

Note that I have LongPathsEnabled set to 1 under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem as described at https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=registry