Introduction
Berkeley DB的游标（Dbc）和关系数据库的游标是类似的——一种可以迭代数据库中的记录的装置。对于重复记录，使用游标来访问他们会更加方便（使用Db的bulk get来得到多条记录是性能最高的做法）。另外，通过游标可以一条条地操作（修改和删除）记录。
在使用游标之前，必须先使用Db::cursor方法打开游标：
int Db::cursor(DbTxn *txnid, Dbc **cursorp, u_int32_t flags);
游标打开后可以通过游标search和write记录。使用完毕后，必须关闭之：
int Dbc::close(void);
例子：
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#include <db_cxx.h>
...
Dbc *cursorp;
Db my_database(NULL, 0);
// Database open omitted for clarity
// Get a cursor
my_database.cursor(NULL, &cursorp, 0);
// Database and cursor open omitted for clarity
// Do something…
if (cursorp != NULL)
cursorp->close();
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Getting Records from DB Using Cursor
打开数据库，并打开此数据库的游标后，就可以使用该游标来search DB来遍历符合条件的记录了。
Dbc::get方法的flag参数可以取多个值，常用的有：
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Flag
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Description
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DB_CURRENT
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返回游标当前所指的key/data
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DB_FIRST
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游标指向第一条记录，并返回该key/data
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DB_GET_BOTH
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只有key/data都匹配才返回该key/data
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DB_GET_RECNO
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返回行号。数据库必须是B树的，且其flag为DB_RECNUM。
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DB_LAST
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和DB_FIRST对应
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DB_NEXT
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游标指向下条记录，并返回下个key/data
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DB_PREV
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和DB_NEXT对应
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DB_MULTIPLE
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返回该key的所有记录。数据库是允许重复记录的。
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例子：
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#include <db_cxx.h>
...
Db my_database(NULL, 0);
Dbc *cursorp;
try {
// Database open omitted for clarity
// Get a cursor
my_database.cursor(NULL, &cursorp, 0);
Dbt key, data;
int ret;
// Iterate over the database, retrieving each record in turn.
while ((ret = cursorp->get(&key, &data, DB_NEXT)) == 0) {
// Do interesting things with the Dbts here.
}
if (ret != DB_NOTFOUND) {
// ret should be DB_NOTFOUND upon exiting the loop.
// Dbc::get() will by default throw an exception if any
// significant errors occur, so by default this if block
// can never be reached.
}
} catch(DbException &e) {
my_database.err(e.get_errno(), "Error!");
} catch(std::exception &e) {
my_database.errx("Error! %s", e.what());
}
// Cursors must be closed
if (cursorp != NULL)
cursorp->close();
my_database.close(0);
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Operating Records Using Cursor
Dbc对象提供了put、del和get方法，通过这些方法我们可以写入、删除和获取记录。
例子：
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#include "stdafx.h"
#include <iostream>
#include <db_cxx.h>
#include <string.h>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
Db db(NULL,0);
u_int32_t oFlags = DB_CREATE; // Open flags;
try
{
db.open(NULL, // Transaction pointer
"my_db.db", // Database file name
NULL, // Optional logical database name
DB_BTREE, // Database access method
oFlags, // Open flags
0); // File mode (using defaults)
db.truncate(NULL,0,0);
float money = 122.45;
char *description = "Grocery bill.";
Dbt key(&money, sizeof(float));
Dbt data(description, strlen(description)+1);
int ret = db.put(NULL, &key, &data, DB_NOOVERWRITE);
cout<<"put data--"<<description<<endl;
ret = db.get(NULL, &key, &data, DB_GET_BOTH);
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
money = 111;
description = "James--------------------";
data.set_data(description);
data.set_size(strlen(description)+1);
db.put(NULL,&key,&data,DB_NOOVERWRITE);
ret = db.get(NULL, &key, &data, DB_GET_BOTH);
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
money = 191;
description = "Mike";
data.set_data(description);
data.set_size(strlen(description)+1);
db.put(NULL,&key,&data,DB_NOOVERWRITE);
ret = db.get(NULL, &key, &data, DB_GET_BOTH);
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
Dbc* cursor;
db.cursor(NULL,&cursor,0);
cout<<"open cursor"<<endl;
while((ret = cursor->get(&key,&data,DB_PREV)) != DB_NOTFOUND)
{
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
}
if (cursor != NULL)
{
cursor->close();
}
money = 191;
description = "Mike";
data.set_data(description);
data.set_size(strlen(description)+1);
db.cursor(NULL,&cursor,0);
cout<<"delete 191..."<<endl;
while((ret = cursor->get(&key,&data,DB_SET)) == 0 )
{
cursor->del(0);
}
if (cursor != NULL)
{
cursor->close();
}
cout<<"after delete 191..."<<endl;
db.cursor(NULL,&cursor,0);
while((ret = cursor->get(&key,&data,DB_PREV)) != DB_NOTFOUND)
{
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
}
if (cursor != NULL)
{
cursor->close();
}
}
catch(DbException &e)
{
cerr<<"DBException:"<<e.what();
}
catch(std::exception &e)
{
cerr<<"DBException:"<<e.what();
}
system("pause");
return 0;
}
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