C++连接mysql数据库时，用getString()方法总是出错

http://www.cnblogs.com/joeblackzqq/p/4332945.html
我按照上面这个链接的教程，用了第二种Connector C++的方法，配置好boost的路径和mysql的路径后，就修改了一下范例的代码：

 #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"

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/tree", "root", "123");
        cout << "连接成功" << endl;
    }
    catch (...)
    {
        cout << "连接失败" << endl;
    }
    sql::Statement* stat = conn->createStatement();
    stat->execute("set names 'gbk'");
    ResultSet *res;
    res = stat->executeQuery("SELECT * FROM testuser");
    while (res->next())
    {

        cout << "Id      :" << res->getInt("id") << endl;
        cout << "Name    :" << res->getString("name") << endl;
        //cout << "Address :" << res->getString("address") << endl;
    }
    if (conn != 0)
    {
        delete conn;
    }
    system("pause");
}

为此我创建了一个数据库tree和表testuser：

create database tree;

use tree;

create table testuser(
id INT(4) PRIMARY KEY,
name CHAR(25),
address varchar(45)
);

desc testuser;

insert into testuser values(1,'Tom','China Beijing'),
(2,'Amy','America NewYork');

select*from testuser;

准备工作都做好了，然后就开始执行：
问题来了：

会不会是空指针引起的

指针的问题，0x000005

string cmd("SELECT * FROM songs WHERE id = " + to_string(song_id) + ";");
res = stmt->executeQuery(cmd.c_str());
if (res->next()) {
name = res->getString("name").c_str();
}

我试过了。加一个c_str();，就可以了。
用MySQL 5.7.23, VS2015, MySQL Connector C++ 1.1.9

注意是 : qs.c_str(); 而不是 qs->c_str(); // 这两个都能通过编译。