What should function returning a reference return on failure?

[Jim Langston]

I'm sure this has been asked a few times, but I'm still not sure.

I want to create a function to simplify getting a reference to a CMap in a
map.

This is what I do now in code:

std::map<unsign ed int, CMap*>::iterato r ThisMapIt = World.Maps.find (
ThisPlayer.Char acter.Map );
if ( ThisMapIt != World.Maps.end( ) )
{
CMap& ThisMap = *((*ThisMapIt). second);
// Work with ThisMap
}

Now, the map number should always be in the map, but I'm a bit pedantic and
like to check for all possible errors. I'd like to create a function to do
this like:

CMap& FindMap( const unsigned int MapNumber )
{
std::map<unsign ed int, CMap*>::iterato r ThisMapIt = World.Maps.find (
ThisPlayer.Char acter.Map );
if ( ThisMapIt != World.Maps.end( ) )
return *((*ThisMapIt). second);
else
// What to return here? A reference can't be null!
}

My alternative is to return a CMap* and return NULL on failure, but I would
rather deal with references so I can use . instead of ->

Any suggestions?

I guess I could return a CMap* and in code do:

CMap* MapP = FindMap( ThisPlayer.Char acter.Map );
if ( MapP != NULL )
{
CMap& ThisMap = *MapP;
// Work with ThisMap
}

if I really have to

[werasm]

Phlip wrote:

Alf P. Steinbach wrote:

* Phlip:

4) pass an optional sentinel object into FindMap, and return that.
5) return an iterator, and check if that == .end().
If the caller passes no sentinel, construct the NullObject one and
return it:

CMap& FindMap(
const unsigned int MapNumber,
CMap const & sentinel = NullMap() );

Oh, my. That's not const-correct. What's the fix?
Leave out the 'const'.

CMap & found = FindMap(2);

Now found refers to the temporary NullMap(), which destructed somewhere
around ;.

Should not work - cannot bind an rvalue (temporary) to a non-constant
reference. I know it compiles under MS VC++ 7.1, but that is erroneous.
This leaves you with this option (or an exception):
....
{
std::map<unsign ed int, CMap*>::iterato r ThisMapIt =
World.Maps.find ( ThisPlayer.Char acter.Map );

if ( ThisMapIt != World.Maps.end( ) )
{
return *((*ThisMapIt). second);
}
else
{
//Same base type as CMap, but you know that!
static NullMap sentinal;
return sentinal;
}
}


--
Phlip
http://www.greencheese.us/ZeekLand <-- NOT a blog!!!

[werasm]

Jim Langston wrote:

I'm sure this has been asked a few times, but I'm still not sure.

I want to create a function to simplify getting a reference to a CMap in a
map. CMap& FindMap( const unsigned int MapNumber )
{
std::map<unsign ed int, CMap*>::iterato r ThisMapIt = World.Maps.find (
ThisPlayer.Char acter.Map );
if ( ThisMapIt != World.Maps.end( ) )
return *((*ThisMapIt). second);
else
// What to return here? A reference can't be null!
}
Instead of you map containing CMap*, I would consider using a map of
type:
std::map<unsign ed int, boost::shared_p tr<CMap> >

FindMap could then return boost::shared_p tr or boost::weak_ptr . The
advantange of this is that the client can store the pointer if he needs
to. He can't use the pointer if invalid (or at least him using it will
cause visible problems). He could also test for validity of the
returned value (No sentinal required). This he could of course do using
normal pointers too (returning NULL), but who's to say the map doesn't
change after finding the item, or who's to say the client doesn't
decide to store the map entry for some reason. I like weak_ptr in this
case because one doesn't want the item to exist after being removed
from the map which weak_ptr enforces, but shared_ptr does not..

Regards,

W
My alternative is to return a CMap* and return NULL on failure, but I would
rather deal with references so I can use . instead of ->

Any suggestions?

I guess I could return a CMap* and in code do:

CMap* MapP = FindMap( ThisPlayer.Char acter.Map );
if ( MapP != NULL )
{
CMap& ThisMap = *MapP;
// Work with ThisMap
}

if I really have to