If the function does not access member data, then it will probably run (though it's still undefined). The problem is that typically functions and data are separate. Functions recieve and extra argument (the this pointer), which points to where the class data exists. So if you call a function in an unitialized pointer, the this pointer sent to the function will be garabage.

So just because you can away with it doesn't mean you should. It's definitely not legal.