Re: hash_map in STL.

Barry <>
Wed, 19 Sep 2007 20:58:56 +0800
Amit Bhatia wrote:

 I am trying to use hash maps from STL on gcc 3.3 as follows:

#ifndef NODE_H
#define NODE_H
#include <ext/hash_map>
#include "node_hasher.h"
class Node;
typedef hash_map<pair<int,int>, Node, Node_Hasher> Loc_Tree;

class Node
{ //everything else;
   Loc_Tree::iterator parent;

And when compiling this does not compile, compiler complains about
forward declaration of Node and says that incomplete type being used in

To correct this, I changed as follows:

#ifndef NODE_H
#define NODE_H
#include <ext/hash_map>
#include "node_hasher.h"
using namespace __gnu_cxx;
class Node
{ //everything else;
   hash_map<pair<int,int>, Node, Node_Hasher>::iterator parent;

typedef hash_map<pair<int,int>, Node, Node_Hasher> Loc_Tree;

I don't know gnu c++ lib,
But I guess most hash_map implementation should go like this

hash_map<Key, Value, ....>

for compatible usage for map, multimap


Now the compiler as expected still complains: error: `std::pair<_T1,
_T2>::second' has incomplete type

I googled for this and it was suggested somewhere to define a base class
(say Base_Node) and then make Node derived class and use Base_Node in
hash_map as follows:

base_bode.h looks like:

#ifndef BASE_NODE_H
#define BASE_NODE_H
class Base_Node{;};

and node.h looks like:

#ifndef NODE_H
#define NODE_H
#include <ext/hash_map>
#include "node_hasher.h"
#include "base_node.h"
using namespace __gnu_cxx;

typedef hash_map<pair<int,int>, Base_Node, Node_Hasher> Loc_Tree;

class Node:public Base_Node
{ //everything else;
   Loc_Tree::iterator parent;


And now it compiles.
However, when I try to insert an object of type Node in Loc_Tree (in
some other .C file), the compiler complains that it wants Base_Node and
I am giving it Node.

How can I fix this problem? If I want to use Node in hash_map, then I
can't use hash_map inside the class declaration.



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