使用c++返回mysql中的varchar类型数据，resultset的getString方法返回该数据报错

1、使用c++返回mysql中的varchar类型数据，resultset的getString方法返回该数据报错,使用try catch语句显示为bad allocation.
2、代码：

#include <iostream>
#include <map>
#include <string>
#include <memory>
#include "mysql_driver.h"
#include "mysql_connection.h"
#include "cppconn/driver.h"
#include "cppconn/statement.h"
#include "cppconn/prepared_statement.h"
#include "cppconn/metadata.h"
#include "cppconn/exception.h"
#include <typeinfo>

using namespace std;
using namespace sql;

int main()
{
    sql::mysql::MySQL_Driver *driver = 0;
    sql::Connection *conn = 0;
    try
    {
        driver = sql::mysql::get_mysql_driver_instance();
        conn = driver->connect("tcp://localhost:3306/jwsystem", "root", "123456");
        cout << "succeed!" << endl;
    }
    catch (...)
    {
        cout << "fail!" << endl;
    }
    sql::Statement* stat = conn->createStatement();
    stat->execute("set names 'gbk'");
    ResultSet *res;
    res = stat->executeQuery("select * from semester");
    while (res->next())
    {
        try{
            string s = "_semester";
            cout << "semester\t:" << res->getString(s) << endl;

        }
        catch (exception e)
        {
            cout<< e.what();
        }

    }
    if (conn != 0)
    {
        delete conn;
    }
    system("pause");
}

3、