Re: [Vxl-maintainers] vil_image_view operator= API change

[Tim C. <Tim...@ma...>]

Hi Amitha,
I agree that failing silently is not terribly satisfactory, as it is 
hard to track down.
Strictly, of course, one should write:

vil_image_view<vxl_byte> im=vil_load(filepath);
if (im.size()==0)
{
    // Uh-oh!
  vcl_cerr<<"I told you not to do that."<<vcl_endl;
  delete_hard_drive();
}

However, we don't all remember to do this.

If one were to abort() on a failure to do a cheap conversion, one would 
have to write

vil_image_view_base_sptr  im_ptr=vil_load(filepath);
if (!im_ptr)
{
  // Failed to load anything...
}
if (im_ptr->is_a()!="vil_image_view<vxl_byte>")
{
  // Wrong type!  Convert or die
}
else
  image=im_ptr;

This is a bit longer, but more explicit.

On the grounds that silent failure is generally a bad thing, I wouldn't 
object to the copy operator aborting if it can't do it easily.  
However, Ian has a deeper knowledge of vil than I, and may have some 
comments.

Tim


Amitha Perera wrote:

>Folks,
>
>I'd like to change the vil_image_view::operator= API to abort() if it
>cannot do a cheap assignment. I've already been bitten multiple times
>by the silent failure where the source and target types don't
>match. One particular example is
>
>  vil_image_view<vxl_byte> im = vil_load( "16-bit image" );
>
>will simply return a null image, and can take a lot of tracking down
>to figure out that there is a type mismatch.
>
>Does someone recall the original justification for silent failure? I
>suggest that if "cheap conversion compatibility" needs to be checked,
>then we create a new method "is_compatible", so that operator= will
>succeed iff is_compatible returns true.
>
>My next email will contain suggestion to make this error a compile
>time error when possible.
>
>Thoughts?
>
>Amitha.
>
>
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-- 
Tim Cootes                                  Senior Lecturer
Imaging Science and Biomedical Engineering  tel   (+44) 0161 275 5146
The University of Manchester                fax   (+44) 0161 275 5145
Manchester M13 9PT , UK                     mailto:t.c...@ma...
                                            http://www.isbe.man.ac.uk/~bim