Scope of variables in case of modules used by modules in Fortran

Question

I\'ve noticed that variables in a (child) module, which are used by a parent modules are accessible in a main program through just the parent module. This is one concept which c

Answer 1

The best thing to do, in my opinion, is to avoid the use of blanket use statements, especially for large and sometimes unwieldy modules. Instead, specify which module entities to inherit via the only keyword, such as:

program main
   use a_mod, only : c, inc_c
   implicit none

   call inc_c
   write(*,*) c
end program main

This works, but it's confusing because a_mod isn't the real owner of c and inc_c. Therefore, you should try to use entities from where they are actually declared, which gives:

program main
   use c_mod, only : c, inc_c
   !   ^ This has changed
   implicit none

   call inc_c
   write(*,*) c
end program main

Now, anybody reading the code has a clear notion of which variables and subroutines are in scope and where they come from.

Finally, this has the added benefit of reducing the risk that you use c without realizing it's actually inhereted from c_mod. This is particularly a problem when not using implicit none!