I've reviewed several similar questions and their answers and nothing I've tried thus far has worked.

I'm starting a Qt project that will be quite large in scope. To keep things better organized, I intend on utilizing subdirectories within my Qt project. Things are not off to a good start.

I am using: Qt Creator 2.8.1 based on Qt 5.1.1

Here is what I've come up with based on previous answers I've seen:

feather.pro
Qt Code: 
  1. QT       += core gui
  2.  
  3. greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  4.  
  5. TARGET = feather
  6. TEMPLATE = app
  7.  
  8. include(gui/gui.pri)
  9.  
  10. SOURCES += main.cpp
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main.cpp
Qt Code: 
  1. #include <QApplication>
  2. #include "gui/mainwindow.h"
  3.  
  4. int main(int argc, char *argv[])
  5. {
  6.     QApplication a(argc, argv);
  7.     MainWindow w;
  8.     w.show();
  9.  
  10.     return a.exec();
  11. }
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gui/gui.pri
Qt Code: 
  1. SOURCES += gui/mainwindow.cpp
  2. HEADERS += gui/mainwindow.h
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gui/mainwindow.h
Qt Code: 
  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. #include <QMainWindow>
  5.  
  6. class MainWindow : public QMainWindow
  7. {
  8.     Q_OBJECT
  9.  
  10. public:
  11.     MainWindow(QWidget *parent = 0);
  12.     ~MainWindow();
  13. };
  14.  
  15. #endif // MAINWINDOW_H
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gui/mainwindow.cpp
Qt Code: 
  1. #include "mainwindow.h"
  2.  
  3. MainWindow::MainWindow(QWidget *parent)
  4.     : QMainWindow(parent)
  5. {
  6. }
  7.  
  8. MainWindow::~MainWindow()
  9. {    
  10. }
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This is about as barebones as it gets. The error received is:

Error: dependent '..\feather\mainwindow.h' does not exist. jom:
C:\Users\Zac\Dev\Qt\build-feather-Desktop_Qt_5_1_1_MSVC2012_32bit-Debug\Makefile
[debug] Error 2 16:12:50: The process
"C:\Qt\Qt5.1.1\Tools\QtCreator\bin\jom.exe" exited with code 2. Error
while building/deploying project feather (kit: Desktop Qt 5.1.1
MSVC2012 32bit) When executing step 'Make'
The error is the same if I use the standard feather.pro:
Qt Code: 
  1. QT       += core gui
  2.  
  3. greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  4.  
  5. TARGET = feather
  6. TEMPLATE = app
  7.  
  8. SOURCES += main.cpp \
  9.         gui/mainwindow.cpp
  10. HEADERS += gui/mainwindow.h
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I don't know why it's dependent on feather/mainwindow.h instead of feather/gui/mainwindow.h. Any advice?