macosxcode
As of this morning, Xcode 6.4 stopped launching on my Mac running OS X 10.10.5 Yosemite.
I tried to launch with
sudo open --new /Applications/Xcode.app
but got
LSOpenURLsWithRole() failed with error -10810 for the file /Applications/Xcode.app
I've reinstalled Xcode, but get the same result. I've tried Disk Utility and antivirus software and my system seems to be clean.
Any ideas how I should proceed, short of reinstalling OS X?
Use RCDefaultApp prefpane. You can get it from http://www.rubicode.com/Software/RCDefaultApp/
Open it, select UTIs and scroll down to com.apple.xcode.project. 
On the right you can change which filetype is associated to which application or disable the association completely.
Edit: another alternative is Magic Launch, which allows the same kind of file association overrides.
Open it, drag an Xcode project file into the pane on the left, set the Default Application on the right to the appropriate version of Xcode.
Did you run sudo xcode-select -switch /Applications/Xcode.app? This message: error: can't exec '/Developer/usr/bin/xcodebuild' is telling you that your system is looking for xcodebuild in `/Developer/usr/bin/' but as of Xcode 4.3 (the one that's installed from the App Store) it's located in the Xcode.app bundle which is in /Applications.
Edit
In a comment to your question you say you've run sudo xcode-select -switch /Applications/Xcode.app. Now run xcode-select -print-path and make sure it says "/Applications/Xcode.app".
Also, you may need to download the command line tools separately. (I don't know from first-hand experience because I'm stuck on Snow Leopard for the moment so I haven't installed Xcode 4.3.)
Best Answer
Make a new user account (admin role) and log out.
If you can start Xcode from Launchpad or Finder in the new account - it's a user level issue. If not, then start with system issues like restart / reinstall latest combo updater or safe boot.