[GRASS-dev] Use of "const", qualified map names

Glynn Clements glynn at gclements.plus.com
Sat Apr 14 19:28:36 EDT 2007

Glynn Clements wrote:

> Which reminds me of another outstanding clean-up task: library
> functions should use "const" where appropriate.
> As it stands, functions which take a non-const pointer but which don't
> actually modify the value are so common that a lot of application code
> tends to just assume that the missing "const" is an oversight and that
> the value won't actually be modified.
> Then it gets bitten by when the function really does modify the value
> (G_find_cell() is a common culprit, as it strips off any @mapset
> suffix from the map name).
> We need to get into the habit of using "const" wherever possible, so
> that the lack of it indicates that the string (or other value) really
> will be modified (rather than simply indicating that someone forgot
> the "const").
> This needs to happen from the bottom up, as once a function declares a
> parameter as "const", it can't pass it to a function which requires a
> non-const parameter (using a type cast can hide a real problem, in the
> case where the function really does modify the argument).

Over the last two days I have gone through libgis and added "const"
qualifiers wherever they were appropriate (as well as a few other
minor clean-ups).

However, this required a subtle API change.

Previously, just about every libgis function which accepted parameters
of the form (char *name, char *mapset, ...) would end up normalising
the name (removing any "@mapset" part). In most cases, this was likely
to be accidental: an artifact of using G_find_file() etc.

Apart from meaning that you couldn't pass a "const char *" for the
name, it could cause problems if you weren't expecting it (certainly,
I've been bitten by it at least once).

Following my changes, the only functions which should strip the
@mapset part from a map name are the G_find_{file,cell,vector}
functions (which return the mapset where the file was found). Anything
else should leave the name intact (but will still honour any @mapset

There is a remote possibility that some module somewhere was actually
relying upon the existing behaviour (rather than using G_find_*() to
explicitly normalise the name). As this would be almost impossible to
track down methodically, all that I can suggest is to be on the
lookout for modules misbehaving when given qualified map names.

On a related note, I've modified the Tk map browser
(lib/gtcltk/select.tcl) to always include the @mapset part in map
names. Previously, it omitted it if the mapset was the current mapset,
which will fail if you have other mapsets ahead of the current mapset
in the search path.

[Note: you can't make this check at the point that the map was
selected, as the mapset path might be modified afterwards. Then,
redisplay would suddenly show the wrong map.]

Glynn Clements <glynn at gclements.plus.com>

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