mysql - C++ 字符串和使用 MySQL

Question

我在将字符串变量传递到 MySQL 连接器时遇到问题。我正在使用 VS2015 和 MySQL Examples 中的标准示例。 下面的代码工作正常

#include "stdafx.h"
#include <iostream>
#include "mysql_connection.h"
#include <cppconn/driver.h> 
#include <cppconn/exception.h> 
#include <cppconn/resultset.h> 
#include <cppconn/statement.h>

using namespace std;

int main()
{

cout << "Running 'SELECT 'Hello World!' AS _message'..." << endl;
try {
    sql::Driver *driver;
    sql::Connection *con;
    sql::Statement *stmt;
    sql::ResultSet *res;
    // Create a connection   

    //string db = "test";
    driver = get_driver_instance();
    con = driver->connect("tcp://127.0.0.1:3306", "user", "user");
    // Connect to the MySQL test database //  
    con->setSchema("test");
    //con->setSchema(db);
    stmt = con->createStatement();
    res = stmt->executeQuery("SELECT 'Hello World!' AS _message");
    while (res->next())
    {
        cout << "\t... MySQL replies: ";
        // Access column data by alias or column name //    
        cout << res->getString("_message").c_str() << endl;
        cout << "\t... MySQL says it again: ";
        // Access column fata by numeric offset, 1 is the first column //    
        cout << res->getString(1).c_str() << endl;

    }
    delete res;
    delete stmt;
    delete con;
}
catch (sql::SQLException &e) {
    cout << "# ERR: SQLException in " << __FILE__;
    cout << "(" << __FUNCTION__ << ") on line " << __LINE__ << endl;  cout << "# ERR: " << e.what();
    cout << " (MySQL error code: " << e.getErrorCode();
    cout << ", SQLState: " << e.getSQLState() << " )" << endl;
}
getchar();
return 0;
}

但是，由于我不想在文件中对数据库模式进行硬编码，因此我尝试使用字符串变量 db 并将其传递给 MySQL 连接器，如下面的代码所示

#include "stdafx.h"
#include <iostream>
#include "mysql_connection.h"
#include <cppconn/driver.h> 
#include <cppconn/exception.h> 
#include <cppconn/resultset.h> 
#include <cppconn/statement.h>

using namespace std;

int main()
{

cout << "Running 'SELECT 'Hello World!' AS _message'..." << endl;
try {
    sql::Driver *driver;
    sql::Connection *con;
    sql::Statement *stmt;
    sql::ResultSet *res;
    // Create a connection   

    string db = "test";
    driver = get_driver_instance();
    con = driver->connect("tcp://127.0.0.1:3306", "user", "user");
    // Connect to the MySQL test database //  
    //con->setSchema("test");
    con->setSchema(db);
    stmt = con->createStatement();
    res = stmt->executeQuery("SELECT 'Hello World!' AS _message");
    while (res->next())
    {
        cout << "\t... MySQL replies: ";
        // Access column data by alias or column name //    
        cout << res->getString("_message").c_str() << endl;
        cout << "\t... MySQL says it again: ";
        // Access column fata by numeric offset, 1 is the first column //    
        cout << res->getString(1).c_str() << endl;

    }
    delete res;
    delete stmt;
    delete con;
}
catch (sql::SQLException &e) {
    cout << "# ERR: SQLException in " << __FILE__;
    cout << "(" << __FUNCTION__ << ") on line " << __LINE__ << endl;  cout << "# ERR: " << e.what();
    cout << " (MySQL error code: " << e.getErrorCode();
    cout << ", SQLState: " << e.getSQLState() << " )" << endl;
}
getchar();
return 0;
}

运行时因未处理的异常而失败

Hello World.exe 中 0x00007FFCBA981F28 处出现未处理的异常:Microsoft C++ 异常:内存位置 0x000000F08CAFCDA0 处的 std::bad_alloc。

我可以看到 MySQL 正在断开连接，因此我假设 MySQL 无法理解作为模式传递的字符串。

我怀疑变量 db 中的编码/终止与“test”不同，因此出现了问题。

任何帮助将不胜感激。

Answer 1

以下代码更改有效。

con->setSchema(db.c_str());

我猜这是 std::string 和 sql::SQLstring 之间转换的问题。

谢谢