R extension memory leak detection question

I am writing C++ program based on R extensions and also try to test the
program with google address sanitizer.

I thought if we don't protect the variable from the allocation API such as
Rf_allocVector, there will be a memory leak. However, the address sanitizer
didn't report it. Is my understanding correct? Or I will see the memory
leak only if I compile R source code with the address sanitizer.

Your question is unclear without an actual example.  It all depends on 
how the variable was created and how you use it.

If your real code is only a few lines, post it here.  Otherwise, please 
put together a minimal working example that contains the essence of what 
you are doing in a few lines.  Check that it compiles, and we can 
provide advice about whether it is doing dangerous things.

Duncan Murdoch