Qt Including Files in Different Directories - c++

Question

So I've read a few posts and articles about how to go about doing this and one option was to do:
#include "../file/file.h"
But when I use this method, it still cannot find the file. The other way was to add the root of the project to the include path.
So here is an example directory:
main_dir
- dir2
-dir3
-header.h
-dir4
-dir 5
-source.cpp
So if I am trying to include header.h from source.cpp, how do I do this?
My .pro file
#-------------------------------------------------
#
# Project created by QtCreator 2015-02-17T12:52:00
#
#-------------------------------------------------
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = project1
TEMPLATE = app
SOURCES += main.cpp\
mainwindow.cpp \
dir4/dir 5/source.cpp \
HEADERS += mainwindow.h \
dir2/dir3/header.h
FORMS += mainwindow.ui

Answer 1

There are many ways to solve the problem.
Use #include "../../dir2/dir3/header.h" in source.cpp.
If you can add main_dir to the list of include directories, (-I<main_dir_path>), then you can use #include "dir2/dir3/header.h" in source.cpp.
If you can add the toplevel directories under main_dir -- main_dir/dir2 and main_dir/dir4 -- to the list of include directories, then you an use #include "dir3/header.h" in source.cpp.