Unresolved external symbols - Qt creator

Question

I must be missing a basic concept with headers and includes because when I attempt to call even the simplest of a function from a separate source file I get an error:

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Answer 1

yes, some time Makefile file is not updated while you change .pro file. So you have to run qmake.

Follow this steps:

• Right click on project Clean / Build menu -> cleanAll
• Right click on project Run qmake / Build menu -> runQmake
• Right click on project Build / Build menu -> build

Pro Advice:

I don't know but sometime Qt is not updating Makefile. so i recomanded to all whenever you add/removing any resource in project or if any changes occur in your .pro file, just Run qmake and build your project(Running qmake do manually to update the path of project, which help to find the mainwindow.obj file).