C++ source code to connect to MariaDB(MySQL).

Mysql.cpp

/*
 * Example to connect to MariaDB(MySQL)
 */
#include <iostream>
#include <mysql/mysql.h>  // require libmysqlclient-dev
#include <string>

using namespace std;

/*
 * [CLASS] Process
 */
class Proc
{
    const char* MY_HOSTNAME;
    const char* MY_DATABASE;
    const char* MY_USERNAME;
    const char* MY_PASSWORD;
    const char* MY_SOCKET;
    enum {
        MY_PORT_NO = 3306,
        MY_OPT     = 0
    };
    MYSQL     *conn;
    MYSQL_RES *res;
    MYSQL_ROW row;

public:
    Proc();           // Constructor
    bool execMain();  // Main Process
};

/*
 * Proc - Constructor
 */
Proc::Proc()
{
    // Initialize constants
    MY_HOSTNAME = "localhost";
    MY_DATABASE = "mysql";
    MY_USERNAME = "root";
    MY_PASSWORD = "7621mizuiko3701";
    MY_SOCKET   = NULL;

}

/*
 * Main Process
 */
bool Proc::execMain()
{
    try {
        // Format a MySQL object
        conn = mysql_init(NULL);

        // Establish a MySQL connection
        if (!mysql_real_connect(
                conn,
                MY_HOSTNAME, MY_USERNAME,
                MY_PASSWORD, MY_DATABASE,
                MY_PORT_NO, MY_SOCKET, MY_OPT)) {
            cerr << mysql_error(conn) << endl;
            return false;
        }

        // Execute a sql statement
        if (mysql_query(conn, "SHOW TABLES")) {
            cerr << mysql_error(conn) << endl;
            return false;
        }

        // Get a result set
        res = mysql_use_result(conn);

        // Fetch a result set
        cout << "* MySQL - SHOW TABLES in `"
             << MY_DATABASE << "`" << endl;
        while ((row = mysql_fetch_row(res)) != NULL)
            cout << row[0] << endl;

        // Release memories
        mysql_free_result(res);

        // Close a MySQL connection
        mysql_close(conn);
    } catch (char *e) {
        cerr << "[EXCEPTION] " << e << endl;
        return false;
    }
    return true;
}

/*
 * Execution
 */
int main(){
    try {
        Proc objMain;
        bool bRet = objMain.execMain();
        if (!bRet) cout << "ERROR!" << endl;
    } catch (char *e) {
        cerr << "[EXCEPTION] " << e << endl;
        return 1;
    }
    return 0;
}

isn't libmysqlclient-dev and the associated <mysql/mysql.h a C rather than C++ library?
Ref: https://dev.mysql.com/doc/refman/5.7/en/c-api-implementations.html

Not working:
$ c++ mysql.cpp
/usr/bin/ld: /tmp/ccF0rOIa.o: in function Proc::execMain()': mysql.cpp:(.text+0x65): undefined reference to mysql_init'
/usr/bin/ld: mysql.cpp:(.text+0xad): undefined reference to mysql_real_connect' /usr/bin/ld: mysql.cpp:(.text+0xcb): undefined reference to mysql_error'
/usr/bin/ld: mysql.cpp:(.text+0x110): undefined reference to mysql_query' /usr/bin/ld: mysql.cpp:(.text+0x129): undefined reference to mysql_error'
/usr/bin/ld: mysql.cpp:(.text+0x167): undefined reference to mysql_use_result' /usr/bin/ld: mysql.cpp:(.text+0x1cc): undefined reference to mysql_fetch_row'
/usr/bin/ld: mysql.cpp:(.text+0x227): undefined reference to mysql_free_result' /usr/bin/ld: mysql.cpp:(.text+0x237): undefined reference to mysql_close'
collect2: error: ld returned 1 exit status

extra/mariadb-libs 10.5.6-3 [installed: 10.4.12-1]
MariaDB libraries

$ LANG=C; sudo pacman -S libmysqlclient-dev
error: target not found: libmysqlclient-dev

@batot1, regarding your linker error, you forgot to link to the MariaDB/MySQL library. You'd have to pass the -lmariadb flag to the c++ command, if I'm not mistaken.