How can I enable the intellisence in KDevelop using C++ and QT?
Firstly, you'll need to provide us with information on what language you're talking about? KDevelop is multi-language IDE. AFAIK IntelliSense availability is depending on plugins enabled.
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edited ok, now when you specified the language ... have a look here :) http://www.kdevelop.org/mediawiki/index.php/FAQ#How_can_I_enable_code_completion.3F
I believe your problem was about the naming - IntelliSense is registered by Microsoft thus most open-source projects tend to name that feature CodeCompletion :)