用C ++连接数据库

Question

In php I create a config file that opens a connection to the database and then I use that file in all my other files in order to open a connection also. But I can't seem to find a way to do the same thing with c++. I can connect to the database but I can't use it as a class because I have a main() inside it and I can't seem to make it work without the main. This is my code:

// Standard C++ includes
#include <iostream>
#include <cstdlib>
#include <string>
using namespace std;

// Include the Connector/C++ headers
#include "cppconn/driver.h"
#include "cppconn/exception.h"
#include "cppconn/resultset.h"
#include "cppconn/statement.h"

// Link to the Connector/C++ library
#pragma comment(lib, "mysqlcppconn.lib")

// Specify our connection target and credentials
const string server   = "localhost";
const string username = "root";
const string password = "";

int main()
{
    sql::Driver     *driver; // Create a pointer to a MySQL driver object
        sql::Connection *dbConn; // Create a pointer to a database connection object
        sql::Statement  *stmt;   // Create a pointer to a Statement object to hold our SQL commands
        sql::ResultSet  *res;    // Create a pointer to a ResultSet object to hold the results of any queries we run

    // Try to get a driver to use to connect to our DBMS
    try
    {
        driver = get_driver_instance();
    }
    catch (sql::SQLException e)
    {
        cout << "Could not get a database driver. Error message: " << e.what() << endl;
        system("pause");
        exit(1);
    }

    // Try to connect to the DBMS server
    try
    {
        dbConn = driver->connect(server, username, password);
    }
    catch (sql::SQLException e)
    {
        cout << "Could not connect to database. Error message: " << e.what() << endl;
        system("pause");
        exit(1);
    }

    stmt = dbConn->createStatement();

    // Try to query the database
    try
    {
        stmt->execute("USE test");

        res = stmt->executeQuery("SELECT * FROM users");

    }
    catch (sql::SQLException e)
    {
        cout << "SQL error. Error message: " << e.what() << endl;
        system("pause");
        exit(1);
    }


    sql::ResultSetMetaData *res_meta = res -> getMetaData();
    int columns = res_meta -> getColumnCount();

    while (res->next())
    {
        for (int i = 1; i <= columns; i++) {
                cout << res->getString(i) << " | " ;
                }
         cout << endl;
    }

    delete res;
    delete stmt;
    delete dbConn;
    return 0;
}

Answer 1

You need to create a class (eg class DBConnector )and declare functions in header file. For example this piece of code can go into one function:

sql::Driver* DBConnector::GetDriverInstance()
{
try
    {
        m_driver = get_driver_instance();
    }
    catch (sql::SQLException e)
    {
        cout << "Could not get a database driver. Error message: " << e.what() << endl;
        system("pause");
        exit(1);
    }
    return m_driver;
}

Here sql::Driver *m_driver is expected to be declared as a member variable of class in header file. You might need to read more about classes and member functions and variables before actually going ahead with code. Other than this, you need to take very good care of memory management. I would suggest you read more and use smart pointers like std::shared_ptr .