Linux C++使用MySQL数据库

这篇笔记主要参考这篇文章: http://zetcode.com/db/mysqlc/

基础配置

安装必要的文件:

sudo apt-get install libmysqlclient-dev

之后终端进入MqSQL, 建立新的数据库和用户, 然后给用户赋予有关的权限:

CREATE database testdb;
CREATE USER user12@localhost IDENTIFIED BY '34klq*';
GRANT ALL ON testdb.* to user12@localhost;

之后在testdb上创建一个表:

USE testdb;

CREATE TABLE userinfo(
					username VARCHAR(30) PRIMARY KEY,
					password VARCHAR(30) NOT NULL,
					wintimes INT,
					failtines INT
);

主要的API

数据库的主要操作就是增删改查, 那么分别说明这几个API.

首先, 需要获取数据库的实例:

MYSQL *con = mysql_init(NULL);

那么, 所有的操作都必须围绕con进行执行. 这相当于一个连接对象.

之后, 需要建立一个链接, 指明数据库和用户:

mysql_real_connect(m_SQLCON, "localhost", "user12", "34klq*",
                            "testdb", 0, NULL, CLIENT_MULTI_STATEMENTS);

指明用户和密码. 如果返回NULL, 则说明失败.

其余的增删改查的语句, 都通过mysql_query执行. 具体直接参考下面的最终代码.

注意一个地方, 每次执行完成语句后, mysql不是立刻提交, 这里面有事物的概念, 不在这里进行讨论. 需要说明的是, 如果一次进行多次操作, 需要单独进行语句提交操作, 代码格式如下:

MYSQL_RES* pRes;
do {
	pRes = mysql_use_result(m_SQLCON);
	mysql_free_result(pRes);
} while(!mysql_next_result(m_SQLCON));

最终的代码

#include 
#include 
#include 
#include 
#include 
#include 
#include 

/*
 * 数据库的链接单例
 * */
class DBServer {
  public:
    static std::shared_ptr<DBServer> getDBServer() {
        if (DBServer::m_pDBServer != nullptr) {
            return m_pDBServer;
        } else {
            std::mutex mtx;
            std::lock_guard<std::mutex> lock(mtx);
            if (!m_OK) {
                // 注意这里无法使用make_shared
                m_pDBServer = std::shared_ptr<DBServer>(new DBServer());
                m_OK = true;
            }
            return m_pDBServer;
        }
    }

	~DBServer() {
        mysql_close(m_SQLCON);
    }

    // 注册用户信息
    bool registerUserInfo(const std::string &username, const std::string &password, std::string &result);

    // 登录信息验证, 成功返回true
    bool signInQuery(const std::string &username, const std::string &password, std::string &result);

    // 更新用户密码
    bool updateUserPassword(const std::string &username, std::string &result);

    // 更新作战的局数, delta正数获胜, 负数失败
    bool updateUserCombatNum(const std::string &username, int delta, std::string &result);

  private:
    explicit DBServer();
    void finishQuery() {
        MYSQL_RES* pRes;
        do {
            pRes = mysql_use_result(m_SQLCON);
            mysql_free_result(pRes);
        } while(!mysql_next_result(m_SQLCON));

    }

    MYSQL *m_SQLCON;

    static std::shared_ptr<DBServer> m_pDBServer;
    static std::mutex m_mtx;
    static bool m_OK;
};

bool DBServer::m_OK = false;
std::shared_ptr<DBServer> DBServer::m_pDBServer = nullptr;

void finish_with_error(MYSQL *con) {
    fprintf(stderr, "%s\n", mysql_error(con));
    mysql_close(con);
    exit(1);
}

DBServer::DBServer() {
    m_SQLCON = mysql_init(NULL);
    if (m_SQLCON == NULL) {
        std::cerr << "can not connect to mysql server\n";
        exit(1);
    }
    if ((mysql_real_connect(m_SQLCON, "localhost", "user12", "34klq*",
                            "testdb", 0, NULL, CLIENT_MULTI_STATEMENTS)) == NULL) {
        finish_with_error(m_SQLCON);
    }
}

bool DBServer::registerUserInfo(const std::string &username, const std::string &password, std::string &result) {
    std::string sql(
        "INSERT INTO userinfo(username, password, wintimes, failtimes) " + std::string("VALUES (\"") + username + "\", \"" + password +
        "\", 0, 0);");
    if (mysql_query(m_SQLCON, sql.c_str())) {
        result = std::string(mysql_error(m_SQLCON));
        return false;
    }

    finishQuery();

    return true;
}

bool DBServer::signInQuery(const std::string &username, const std::string &password, std::string &result) {
    std::string sql("SELECT username, password FROM userinfo WHERE username = \"" + username + "\" AND password = \"" +
                    password + "\";");
    if (mysql_query(m_SQLCON, sql.c_str())) {
        result.clear();
        result += std::string(mysql_error(m_SQLCON));
        return false;
    }
    auto res = mysql_store_result(m_SQLCON);
    int num_fields = mysql_num_fields(res);
    if (num_fields <= 0) {
        result = std::string("Query error\n");
        return false;
    }

    MYSQL_ROW row = mysql_fetch_row(res);
    if (!row) {
        result = std::string("username: " + username + " does not exist!");
        return false;
    }
    auto username_ = std::string(row[0]);
    auto password_ = std::string(row[1]);
    if (username != username_) {
        result = std::string("username: " + username + " does not exist!");
        return false;
    }
    if (password_ != password) {
        result = std::string("password error!");
        return false;
    }

    finishQuery();

    return true;
}

bool DBServer::updateUserPassword(const std::string &username, std::string &result) {
    return false;
}

bool DBServer::updateUserCombatNum(const std::string &username, int delta, std::string& result) {
    std::string sql("SELECT username FROM userinfo WHERE username = \"" + username + "\";");
    if (mysql_query(m_SQLCON, sql.c_str())) {  // 确认用户确实存在
        result.clear();
        result += std::string(mysql_error(m_SQLCON));
        return false;
    }

    finishQuery();

    sql.clear();
    std::string num;
    num.clear();
    if (delta > 0) {  // 胜利的次数
        sql = std::string(
                  "UPDATE userinfo SET wintimes = wintimes + " + std::to_string(delta) + " where username = \"" + username + "\"");
    } else {  // 失败了
        sql = std::string(
                  "UPDATE userinfo SET failtimes = failtimes + " + std::to_string(-delta) + " where username = \"" + username + "\"");
    }

    if (mysql_query(m_SQLCON, sql.c_str())) {
        result.clear();
        result = std::string("update failed, ") + std::string(mysql_error(m_SQLCON));
        return false;
    }

    finishQuery();

    return true;
}

int main() {
    auto it = DBServer::getDBServer();
    std::string res;
    std::cout << it->signInQuery("tiger", "123", res) << std::endl;
    std::cout << res << std::endl;
    res.clear();
    std::cout << it->updateUserCombatNum("tiger", 1, res);
    std::cout << res << std::endl;
    res.clear();
    std::cout << it->registerUserInfo("lion", "345", res) << std::endl;
    std::cout << res << std::endl;
    return 0;
}