Qt Creator can't find headers (says: “No such file or directory”)

Question

How can I tell Qt Creator 2.4.1 (based on Qt 4.7.4 32-bit) where to look by default for header files?

When I open a C file in Qt Creator and say

Answer 1

Or must I create an entire project every time I want to edit a standalone C++ file?

No, you can include standalone file in existing project by just one right click on root of the project tree - context menu - add existing file. You can assign hot key for that, as well as for removing file from the project, so it will be fast and easy to use your favorite editor and its environment. You can look at it as you just opening file. Unfortunately there are no command-line arguments for assigning a default project (and its environment) for a standalone file for using it by double click in a file manager.

Hope it helps.