Pointers casting

[ami]

Hello,

I've got the following code looping over a map :-

...
key = "Key1";
val = "val1";
p = months.find(key);

if(p != months.end()) // found
{
for (months_type::const_iterator it = months.begin();
it != months.end(); ++it)
{
if ((it->first == key) && (it->second->counterpartyCode == val))
{
Here I want to point the pointer to the correct object and update it accordingly. I tried casting as follows, but this isn't working.

//CounterpartyDetails* pExisting =
// (CounterpartyDetails *)(it->second->cashCcy);
//pExisting->cashCcy += 300;
//break;

I know I can do: it->second->cashCcy +=300; but was hoping to find a nice way...
}
}

Can anyone please help ?

[jonnin]

what is cashCcy?
if its a CounterpartyDetails, you need:

CounterpartyDetails* pExisting = &((it->second->cashCcy));


alternately, if second is a *CounterpartyDetails, you just need

CounterpartyDetails* pExisting = (it->second);

[ami]

Thanks for the information.

I forgot to say that when creating the objects, I'm doing :-

...
CounterpartyDetails *ptr = new CounterpartyDetails();
ptr->Name = name;
ptr->cashCcy = ccy;
...
and then insert into a map as follows :-

myMap.insert(value_type(name, ptr));
...
And again :-

...
ptr = new CounterpartyDetails();
ptr->Name = name;
ptr->cashCcy = ccy;
...
and then insert into a map as follows :-

myMap.insert(value_type(name, ptr));


The thing is, I keep on creating objects using the same mechanism, i.e. ptr keeps on moving to the next object obviously.

After creating all objects is completed, how can I change ptr to point to a specific object created, e.g. how would I change ptr to point to the second object created without a loop ?

Thanks.

[jonnin]

um, however you normally get to that object. Map.extract or whatever function gives you the inserted node back, you take the address of it if its data or just use it if its already a pointer. I guess I don't know what you are confused about.

[Peter_APIIT]

His question is in a sequence of map after insert ptr. The ptr should point to last element but in case he want change to second element in map.

Am i correct.


For instance, you five element in a map but u want point to map.

Pointer offset. *(ptr - 3);

I don't know this is correct or not.Please correct if i wrong.

[ami]

Yes, thank you. That's exaclty my question. If I want to access the second or third object in the map via the pointer how would I do that ? Obviously, after the last insert, the pointer points to the last element.

Does this make sense ?

[nspils]

It runs counter to the concept of a Map to be trying to access "by a pointer" and by pointer arithmetic, because a Map stores Pairs of Key and Element. It is organized to find by searching for the Key and returning the Element. While you can have a sorted map it doesn't need to be. What motivates your design to be able to return something by pointer arithmetic?

[Danny]

I agree with nspils. Your design is trying to force maps to do something they weren't design to. Even if that's possible, it's convoluted, inefficient and bug-prone. Maps, as all STL containers, are value sequences. They usually contain objects, not pointers. Maps in particular are made to store pairs, and access them by the pair template, or using map iterators.

[Razee Marikar]

I'm not exactly sure that I get your problem, but try this:

CounterpartyDetails* pExisting =
 (CounterpartyDetails *)(it->second);

[jonnin]

or pull it out of the map and take its address. But there is no concept of "second" item here, right? Even with the iterator they will not be in the order of insertion (right?). Im not 100% sure, I have not actually used this data structure. But the safe way to go is iterate to the Nth item and take its address.

If it turns out that you want the items by order of insertion, your best bet is simply going to be something like add a counter to the data type and iterate over the map (counter productive, but perhaps you will access via map 99% of the time but once in a while, you need them in the orig ordering???) or you can maintain a list of pointers to them in the inserted order (ugly to create, but once done, you are good). Here, you iterate the map and extract the item that matches what you just put in and take its address, place into a simple list or vector or whatever you see fit.