file io - How do you handle resources in MATLAB in an exception safe manner? (like "try ... finally")

Question

Since there is no finally clause to the try-catch block in MATLAB, I find myself writing lots of code like the following:

fid = fopen(filename);
if fid==-1
    error('Couldn''t open file');
end
try
   line = getl(fid);
catch ME
   fclose(fid);
   rethrow ME;
end
fclose(fid);

I find having the fclose function in two places ugly and error prone.

Is there a better way for doing this?

Answer 1

I would suggest checking out ONCLEANUP objects. They allow you to automatically run code on exit from a function (more specifically, when the ONCLEANUP object is cleared from memory). Loren from The MathWorks discusses this in one of her blog posts here. If you place your above code in a function, it might look something like this:

function data = load_line(filename)
  data = [];
  fid = fopen(filename);
  if fid == -1
      error('Couldn''t open file');
  end
  c = onCleanup(@()fclose(fid));
  data = getl(fid);
end

Even if the call to GETL throws an exception, the ONCLEANUP object will still be cleared from memory on return from the function load_line, thus ensuring the file gets closed.