In this example, it's not clear what kind of database was used to construct the db_map
. However, the sequence 256, 1, 257, 2 ... is exactly the sequence I would expect for a memcmp
sorted BTree on a little-endian machine.
db_map
provides a std::map
interface to a Berkeley DB BTree. However, underneath, the key comparison function for db_map
is defined by the BTree comparison function.
From the Db::set_bt_compare
documentation:
If no comparison function is specified, the keys are compared lexically, with shorter keys collating before longer keys
To the database, a key is nothing more than a sequence of bytes. If no key comparison function is specified, memcmp
is used to compare keys. If you want to iterate in the standard numeric order, assign a comparison function using Db::set_bt_compare
:
#include <iostream>
#include <dbstl_map.h>
#include <db_cxx.h>
using namespace dbstl;
int key_compare(DB* db, const DBT* a, const DBT* b, size_t* u)
{
int i,j;
if (0 == a->size || 0 == b->size)
{
return 0;
}
std::memcpy(&i, a->data, sizeof(i));
std::memcpy(&j, b->data, sizeof(j));
return i-j;
}
int main()
{
DbEnv* env = new DbEnv(DB_CXX_NO_EXCEPTIONS);
env->open("/home/centinela", DB_CREATE|DB_INIT_MPOOL, 0);
Db* db = new Db(env, DB_CXX_NO_EXCEPTIONS);
db->set_bt_compare(&key_compare);
db->open(NULL, "test.db", NULL, DB_BTREE, DB_CREATE, 0);
typedef dbstl::db_map<int,std::string> map_type;
map_type* mp = new map_type(db, env);
for (int i = 1; i <= 500; ++i)
{
mp->insert(std::pair<int,std::string>(i,"t"));
}
map_type::const_iterator it;
for (it = mp->begin(); it != mp->end(); ++it)
{
std::cout << it->first << std::endl;
}
delete db;
delete env;
}
Note that my example uses the four parameter compare function from BDB 6.0. Comparison functions for BDB 5.0 do not include the last parameter. See here.