c++ - MySQL 结果集包含 1 行，但调用 getstring 导致访问冲突

Question

我目前正在学习 C++，并正在慢慢取得进展，但遇到了 MySQL 选择和准备语句的问题。

我有一个链接到库的应用程序，该应用程序正在调用库函数，如下所示:

try
        {
            DynamicConfig dynamicConfig;
            string value = "";
            dynamicConfig.getValueFromDBConfig("Test Group", "TestName", &value);
            stringstream logStream;
            logStream << "Dynamic Config Value: " << value;
            bitsLibrary.writeToLog(logStream.str());
        }
        catch (sql::SQLException ex)
        {
            cout << "SQL Exception: " << ex.what() << endl;
        }
        catch (exception ex)
        {
            cout << "General Exception: " << ex.what() << endl;
        }

在 DynamicConfig 类的库中，我有 getValueFromDBConfig 方法，其中包含以下内容:

bool DynamicConfig::getValueFromDBConfig(string groupName, string sectionName, string *value)
{
    //Connection *conn = NULL;
    PreparedStatement *statement = NULL;
    ResultSet *resultset = NULL;
    try
    {
        DBManager dbManager;
        Connection *conn = dbManager.getDriverConnection();
        statement = conn->prepareStatement("SELECT * FROM config_items, config_groups WHERE config_groups.Name=? AND config_items.Name=? AND config_groups.id = config_items.ConfigGroupID");
        statement->setString(1, groupName.c_str());
        statement->setString(2, sectionName.c_str());
        resultset = statement->executeQuery();
        cout << "Found " << resultset->rowsCount() << " rows" << endl;
        if (resultset->rowsCount() > 0)
        {

            while (resultset->next())
            {
                cout << "Value: " << resultset->getString("Value") << endl;
                *value = resultset->getString("Value");
            }
            delete conn;
            delete statement;
            delete resultset;
            return true;
        }
        else
        {
            bitsLibrary.writeToLog("No rows were returned", "DynamicConfig", "getValueFromDBConfig");
        }
    }
    catch (SQLException ex)
    {
        stringstream logStream;
        logStream << "Failed to get DB config value. SQL Exception: " << ex.what();
        bitsLibrary.writeToLog(logStream.str(), "DynamicConfig", "getValueFromDBConfig");
        throw ex;
    }
    catch (exception ex)
    {
        stringstream logStream;
        logStream << "Failed to get DB config value. General Exception: " << ex.what();
        bitsLibrary.writeToLog(logStream.str(), "DynamicConfig", "getValueFromDBConfig");
        throw ex;
    }
    return false;
}

它似乎成功执行了查询，因为它将 1 rows 打印到控制台，但是，当我调用 resultset->getstring("Value") 时，它会抛出错误:

Exception thrown at 0x00007FFF7E831EBA (msvcp140d.dll) in TestApp.exe 0xC0000005. Access violation reading location 0xFFFFFFFFF.

我不明白为什么这不起作用，它有一行，而且我正在查看的数据库表肯定有名为 Value 的列。

Answer 1

最后终于明白了。我需要按如下方式运行 getstring

*value = resultset->getString("Value").c_str()

我不明白为什么我需要这样做，我使用的MySQL示例代码来自 https://dev.mysql.com/doc/connector-cpp/en/connector-cpp-examples-results.html没有显示这一点，所以如果有人可以阐明这一点，那就太好了。