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Let's take a step back and get onto the same ground of discussion before we jump into C++ vs Obj-C. The Mac Cocoa and iOS Cocoa Touch frameworks are written in Objective-C. You NEED to write an Obj-C application to interface to these frameworks. So if you intend to write an application for an iOS device or a Mac, you will have to do it in Obj-C.
Now, here is no reason that you can't write a portion of your application in Obj-C while talking to the framework and use C++ for non-framework related work, this can be confusion especially if you are just beginning to program.
The most common place where I see this happening if in a game or something where you use Obj-C to do all your device/Mac related UI stuff, and then have a physics engine like Box2D which uses C++. You can mix-and-match them without any problems. For that matter, you could even mix-and-match Chipmunk a C-based physics engine.
That being said, you cannot write a purely C++ application and make attempts to use the Apple frameworks, it just doesn't work.
Now as far as fast and secure are concerned. Think about it for a second, the entire framework that runs your Mac or iOS device is based on Obj-C, so writing an application that uses the same language means that it will fit in better, be more secure since you're not finding strange ways to interface to the framework and most likely faster since you aren't "translating" anything across languages..
Having said all that, you will be doing yourself a great favor by sticking with Obj-C if your intention is to write an application for the Mac or iOS device..