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

[BigBrian]

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.

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?

Throw an exception.

[Alf P. Steinbach]

* Jim Langston:

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

If the function returns a reference, it's guaranteeing that if it
returns, the result is a valid reference.

You have the choice of using (1) a reference to some special object
denoting "no object" or "failure", or (2) throwing an exception.

(2) is most clean, most reliable wrt. client code checking, and may help
avoid constructing a large dummy object.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Andre Kostur]

On Mon, 08 May 2006 07:50:00 -0700, 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.

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

That's one of your choices. There are three ways that I can think of:

1) Return a pointer and check for NULL.
2) Throw an exception and catch it somewhere
3) Return a reference to a "NULL CMap", that is a special instance of your
CMap object representing that no map exists.

[Jim Langston]

"Alf P. Steinbach" <al***@start.no > wrote in message
news:4c******** *****@individua l.net...

* Jim Langston:

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

If the function returns a reference, it's guaranteeing that if it returns,
the result is a valid reference.

You have the choice of using (1) a reference to some special object
denoting "no object" or "failure", or (2) throwing an exception.

(2) is most clean, most reliable wrt. client code checking, and may help
avoid constructing a large dummy object.

I generally only like throwing exceptions on genuine errors, but you know
what? Not finding the CMap in the map is a genuine error, because it should
be there.

Thanks! I'll do that.

[Howard Hinnant]

In article <yU***********@ fe05.lga>,
"Jim Langston" <ta*******@rock etmail.com> wrote:

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?

One school of thought is that if the missing map represents a run time
failure that you're anticipating (lack of memory, full disk, bad user
input, etc.) then an exception is appropriate. However if the missing
map represents a programming error, an assert may be more appropriate.

Of course for programs that must keep running no matter what, an assert
is rarely appropriate.

If you throw an exception, will you know what to do with it when you
catch it? If you can correct the error on catch, then an exception
sounds like the way to go. If on catch you're just going to say "I've
got a bug" and terminate, then an assert might actually help you find
the error earlier because it will be reporting it earlier (before stack
unwinding).

-Howard

[Phlip]

Jim Langston wrote:

I generally only like throwing exceptions on genuine errors, but you know
what? Not finding the CMap in the map is a genuine error, because it
should be there.

If it's a _programmer_ error, put assert(false) at the bottom of the
function.

If it's an unpreventable _user_ error, throw. (So also throw if that
assert(false) gets optimized away. And pick a better argument than false.)

To finish Andre's list:

4) pass an optional sentinel object into FindMap, and return that.

If the caller passes no sentinel, construct the NullObject one and return
it:

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

--
Phlip
http://c2.com/cgi/wiki?ZeekLand <-- NOT a blog!!!

[Phlip]

> 4) pass an optional sentinel object into FindMap, and return that.

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?

--
Phlip
http://c2.com/cgi/wiki?ZeekLand <-- NOT a blog!!!

[Jim Langston]

"Alf P. Steinbach" <al***@start.no > wrote in message
news:4c******** *****@individua l.net...

* Jim Langston:

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

If the function returns a reference, it's guaranteeing that if it returns,
the result is a valid reference.

You have the choice of using (1) a reference to some special object
denoting "no object" or "failure", or (2) throwing an exception.

(2) is most clean, most reliable wrt. client code checking, and may help
avoid constructing a large dummy object.

Dang, there's one problem with the try...catch.

try
{
CMap& ThisMap = FindMap( MapNumber );
}
catch ( int )
{
LogError("Could not find map");
}

That ThisMap is only going to exist during the lifetime of the try block.
And I can't create it outside the block because it's a reference and has to
be initialized.

Now this means I'll have to put whole blocks of code inside the try block,
but I don't want to catch errors in a block for all the code, and a lot of
the code should execute anyway even if they can't find the map.

That is, this code won't compile:

int i = 1;
try
{
int& j = i;
}
catch (...)
{
}

std::cout << j << std::endl;

[Alf P. Steinbach]

* Phlip:

4) pass an optional sentinel object into FindMap, and return that.

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'.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?