After studying symbianosunit, i understood that we have to modify testdriver.h in order to test our application.

I mean creating new class for every test function in testheader.h and overriding its run function.

Also if i clean my project, testdriver.h gets deleted.

So is it meant to be created automatically??

I came to know that some pearl script is handling this..but is there any way to disable this?
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No, you do not change the TestDriver file itself. That one gets created from other files, by the perl scripts you mentioned.
The idea is that you write the test cases and that the perl script takes care of creating the necessary files to really run your tests.
The perl script will extract form your file all the functions that start with "test"... the best is to follow the tutorial to understand what you have to change
Since C++ does not really allow introspection, you would need to register all your test functions, which can easily become a very painful task. Thus, the perl scripts.
If you want to disable the perl scripts you would need to disable it in the bld.inf file (where the make file is included, if I remember correctly... was quite a while since I lust touched this part and I ave made quite a few changes in my updated version of the framework), but then again, why would you like to do that, since the perl script is exactly what takes away a lot of the work.
Yes, the script does not check whether you have or haven't changed anything in your C++ file, and therefore (unnecessarily) recreate the TestDriver, but this is not really a too big problem, since you will recompile you test cases (and run them) when you have made changes.