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Hash_map with custom class pointers

Posted on 2004-04-12
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Medium Priority
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Last Modified: 2008-02-01
Hi,

I have two classes
Id and ItemHandle

I am trying to use hash_map to instantiate them and run inserts and find. The code I looked up

class foo{
public:
hash_map<Id*, ItemHandle*> *handles2;
}
foo(){
handles2 = new hash_map<Id*, ItemHandle*>;
}

foo::somecommand(Id* something, ItemHandle* someHandlePtr){
//insert
handles2[something->getId()] = someHandleptr;

//find
ItemHandle *temp = handles2->find(something->getId();

//delete


}

but when I compile, i get the following error (for the insert)

file.cpp:70: invalid
   types `__gnu_cxx::hash_map<Id*, ItemHandle*,
   __gnu_cxx::hash<Id*>, std::equal_to<Id*>,
   std::allocator<ItemHandle*> >*[Id*]' for array
   subscript

  Thanks.

-Edward
0
Comment
Question by:edwardt
  • 5
7 Comments
 

Author Comment

by:edwardt
ID: 10807671
If I do a
handles2->insert(something->getId(), someHandlePtr); i get the following error:

file.cpp:71: no
   matching function for call to `__gnu_cxx::hash_map<Id*,
   ItemHandle*, __gnu_cxx::hash<Id*>,
   std::equal_to<Id*>, std::allocator<ItemHandle*> >::
   insert(Id*, ItemHandle*&)'
/usr/include/c++/3.2/ext/hash_map:171: candidates are:
   std::pair<__gnu_cxx::hashtable<std::pair<const _Key, _Tp>, _Key, _HashFcn,
   std::_Select1st<std::pair<const _Key, _Tp> >, _EqualKey, _Alloc>::iterator,
   bool> __gnu_cxx::hash_map<_Key, _Tp, _HashFcn, _EqualKey,
   _Alloc>::insert(__gnu_cxx::hashtable<std::pair<const _Key, _Tp>, _Key,
   _HashFcn, std::_Select1st<std::pair<const _Key, _Tp> >, _EqualKey,
   _Alloc>::value_type&) [with _Key = Id*, _Tp =
   ItemHandle*, _HashFcn = __gnu_cxx::hash<Id*>,
   _EqualKey = std::equal_to<Id*>, _Alloc =
   std::allocator<ItemHandle*>]

0
 
LVL 4

Accepted Solution

by:
bkfirebird earned 1200 total points
ID: 10807825
first declare .....
struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return strcmp(s1, s2) == 0;
  }
};

and then use hash_map as ......
hash_map<const char*, int, hash<const char*>, eqstr> *handles2;
0
 
LVL 44

Assisted Solution

by:Karl Heinz Kremer
Karl Heinz Kremer earned 300 total points
ID: 10807880
What type does your method getId() return? If it's Id, then this is your problem. You defined the hash_map with Id* and not with Id.
0
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Author Comment

by:edwardt
ID: 10817191
bkfirebird
so you're saying that i need to initialize my hash_map with types const char* and int, instead of my custom classes?

khkremer
getId() returns Id*

-Edward
0
 

Author Comment

by:edwardt
ID: 10817301
I still get the same errors when I try
handles2[getId()] = stuff;

or

handles2->insert(getId(), stuff);
0
 

Author Comment

by:edwardt
ID: 10818782
I made the eqstr stuff and now I have

hash_map<const char*, Receiver*, hash<const char*>, eqstr> handles2;
       
where my hash function is based on a const char *

I have a function that I use to add things into my hash table, which is called roughly like

myfunc(receiver1, receiver1->toString().c_str());
myfunc(receiver2, receiver2->toString().c_str());

void Foo::myfunc(Receiver *receiver, const char *str)
{
     hash_map<const char*, Receiver*, hash<const char*>, eqstr>::iterator i;


    /*
    for (i=handles2.begin(); i!=handles2.end(); i++)
    {
        const char* key = i->first;
        cout << key  << ": is what i have" <<endl;

    }
    */
   
    i = handles2.find(str);

    if (i != handles2.end())
    {
        cout<<"ERROR - already-registered address "<<str<<endl;
        assert(false);
    }
    addressBook[str] = receiver;

}

The problem is that when I pass in a new string, then my hash table's key entry automatically changes!!!!! :( ARGH!!!! That means if my first pair is

"hello", Recever1

and my second pair that i pass in is

"hello2", Receiver2

then it changes my original hash value from "hello" to "hello2"......

Thanks.

-Edward
0
 

Author Comment

by:edwardt
ID: 10819141
Hi,

The basic problem I'm having is that I need to store hashes as
hashtable[someitem->toString().c_str()] = value;

where the result of .c_str() is lets say, "mykey"
but i need to access them elsewhere using something like

hashtable.find("mykey");

Right now, it doesn't seem to work, because my hash table (i think) stores the address of c_str() instead of a literal (which I want). Anyway to convert string to a string literal :P?

Thanks.

-Edward
0

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