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Not exactly.

You'll have to make two separate projects in Xcode, but since both use Objective C and similar programming paradigms and architecture, you'll be able to have pretty similar parts in how the back end works.

As for the front end (GUI), of course people expect different things depending on their method of input (mouse/trackpad or touch), so I wouldn't do the exact same thing in both. You'll need to separate projects since Mac apps use the Cocoa UI framework while iOS apps use Cocoa Touch. Both frameworks share a lot in common, but they're not unified as a single framework.

But still, if you have experience making iOS apps, it should be pretty easy for you to make Mac apps using Xcode/Cocoa and vice-versa.
 
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