[petsc-users] VecGet/RestoreArray and related functions

[Mohammad Mirzadeh]

Hi guys,

I am having a conceptual problem trying to design my vector wrappers for my
application. The problem is some of vector functionality in PETSc require
extra function calls which makes it hard to design a good wrapper. One
example that I encountered was when trying to write the destructor for my
vector object. You see, for example, the following code is not good enough

Vector::~Vector(){
VecDestroy(v_local);
}

This, in itself wont work since all calls to VecDestroy() should be made
before PetscFinalize() and this wont happen if you define a Vector in your
main function since the destructor is called upon exit ... So, I had to fix
this by actually having a destructor function memer that I could call
explicitly before PetscFinalize() or create the Vector object on the heap
so that I can manually invoke the destructor via delete operator!

Another example I'm struggling with is the following: I am trying to write
accessors so that I can do the following:

Vector a (PETSC_COMM_WORLD, PETSC_DECIDE,10); // make a petsc vector of
global length 10 using my own wrappers
for (int i=0; i<a.GetLocalSize(); i++){
a(i) = i;
}

For this to work, I am using the VecGetArray() and VecRestoreArray()
functions. The problem is you cannot easily use them inside the accessor
which basically looks something like:

inline double& Vector::operator()(int i){
return ptr_to_dataArray[i];
}

since you need to call to VecRestoreArray after the assignment which means
after return!. The simple fix to this is to call the VecGetArray() and
VecRestoreArray() in the main function where you are putting in the values
but this does not look very neat to me ... so I ended up putting the
VecGetArray() inside the constructor and VecRestoreArray() inside the
destructor to automate the process.

Now I have the following two questions. 1) Is this safe? As far as I could
understand from the manual, VecGetArray() "returns" a pointer to internal
data stored on this processor. Then I guess VecRestoreArray() is there to
somehow nullify the pointer to prevent accidental use of pointer? Since all
pointers all private members of my class I assume this is safe then unless
these two functions are actually doing some extra work?

2) Is there any better way of writing these wrappers and get around the
problem of having to call "supplemental" functions safely?

Sorry if my lack of knowledge is causing all these confusion.

Thanks,
Mohammad
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