ResultSet->getString() code

[tai a]

mysql> show variables like '%char%';
+--------------------------+---------------------------------------+
| Variable_name | Value |
+--------------------------+---------------------------------------+
| character_set_client | gbk |
| character_set_connection | gbk |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | gbk |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | D:\YJF\YJFMySQL\mysql\share\charsets\ |
+--------------------------+---------------------------------------+
8 rows in set (0.00 sec)

mysql> use ygx;
Database changed
mysql> show create database ygx;
+----------+--------------------------------------------------------------+
| Database | Create Database |
+----------+--------------------------------------------------------------+
| ygx | CREATE DATABASE `ygx` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+--------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table test;
+-------+-----------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-----------------------------------------------------------------------------------------------------------------------------+
| test | CREATE TABLE `test` (
`姓名` varchar(20) DEFAULT NULL,
`年龄` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-----------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> use ygx;
Database changed
mysql> select * from test;
+------+------+
| 姓名 | 年龄 |
+------+------+
| 张三 | 20 |
+------+------+
1 row in set (0.00 sec)




int main()
{
sql::mysql::MySQL_Driver *driver = NULL;
sql::Connection *conn = NULL;
sql::Statement *stmt = NULL;
sql::ResultSet *res = NULL;
try
{
driver = sql::mysql::get_mysql_driver_instance();
conn = driver->connect("tcp://localhost:3306/ygx", "root", "123456");
cout << "连接成功！" << endl;
}
catch (const std::exception&)
{
cout << "连接失败！" << endl;
goto END;
}
try
{
stmt = conn->createStatement();
stmt->execute("set character set gbk");
/* stmt->execute("set character_set_results= gbk");
stmt->execute("set character_set_client=gbk");
stmt->execute("set character_set_connection= gbk");
*/ res = stmt->executeQuery("select * from test where 姓名='张三'");

}
catch (const std::exception&)
{
cout << "查询失败！" << endl;
goto END;
}
cout << "查询成功！" << endl;
try
{
if (res->next())
{
string str;
// string str1 = string_to_utf8(str);
str = res->getString(1).c_str();
// str1 = utf8_to_string(str);
}
}
catch (const std::exception&)
{
cout << "显示失败！" << endl;
goto END;
}
END:
delete res;
delete stmt;
delete conn;
return 0;
}


str = res->getString(1).c_str();
str=张三

str = res->getString("张三").c_str();
str=20

why???