C++ Thread worker and mysql connections

// thread example
// g++ main.cpp  -std=c++11 -pthread
// mysql
// apt-get install libmysql++
// apt-get install libmysql++-dev
// compile
// g++ -o start main.cpp -std=c++11 -pthread -L/usr/include/mysql -lmysqlclient -I/usr/include/mysql
//
#include <iostream>       // std::cout
#include <thread>         // std::thread
// sleep
#include <unistd.h>
// mysql
#include <mysql.h>
// vector
#include <vector>

using namespace std;

vector < vector < string >> select() {
    MYSQL * connect, mysql;
    MYSQL_RES * result;
    int query_state;
    // init mysql
    connect = mysql_init( NULL );
    if( !connect ) {
        cout << "MySQL Initialization failed";
        //return 1;
    }
    connect = mysql_real_connect( connect, "localhost", "root", "toor", "SMTP", 3306, NULL, 0 );
    if( connect ) {
        cout << "connection Succeeded\n";
    } else {
        cout << "connection failed";
    }
   
    // select
    MYSQL_RES * res_set;
    MYSQL_ROW row;
    MYSQL_FIELD * field;
    mysql_query( connect, "select id,email_to,resendtime,send from messages;" );
    unsigned int i = 0;
    res_set = mysql_store_result( connect );
    unsigned int numrows = mysql_num_rows( res_set );
    int num_fields = mysql_num_fields( res_set );
    try {
        while((( row = mysql_fetch_row( res_set ) ) != NULL ) ) {
            cout << "Row " << row[ i ] << " " << row[ i + 1 ] << " " << row[ i + 2 ] << endl;
        }
    } catch( std::exception & e ) {
        cout << "Error pointer " << endl;
    }
    mysql_free_result( res_set );
    mysql_close( connect );
}

void foo()
{
    while( 1 ) {
        cout << "Hello from thread" << endl;
        sleep( 1 );
        // select from thread !!!!!!!
        select();
    }
}

void bar( int x )
{
    while( 1 ) {
        cout << "Hello from thread " << x << endl;
        sleep( 1 );
    }
}

int main()
{
    std::thread t1( foo ); // spawn new thread that calls foo()
    std::thread t2( bar, 0 ); // spawn new thread that calls bar(0)
   
    std::cout << "main, foo and bar now execute concurrently...\n" << endl;
   
    // synchronize threads:
    //t1.join();                // pauses until first finishes
    //t2.join();               // pauses until second finishes
   
    while( 1 ) {
        std::cout << "Main thread looooop \n";
        sleep( 1 );
        // select from main
        //    select();
    }
   
    return 0;
}

#include "db.h"

DB::DB() { }

bool DB::Connect( string host = "localhost", string user = "root", string pass = "toor", string db = "BreakermindSMTP", int port = 3306 ) {
    // init mysql
    connect = mysql_init( NULL );
    if( !connect ) {
        cout << "MySQL Initialization failed";
        return 0;
    }
    connect = mysql_real_connect( connect, host.c_str(), user.c_str(), pass.c_str(), db.c_str(), port, NULL, 0 );
    if( connect ) {
        cout << "Connection Succeeded\n";
        return 1;
    } else {
        cout << "Connection failed";
        return 0;
    }
}

vector < vector < string >> DB::Select( string txt = "Row " ) {
    // vectors
    vector < vector < string >> messages;
    // select
    MYSQL_RES * res_set;
    MYSQL_ROW row;
    mysql_autocommit( connect, false );
    //mysql_query (connect,"BEGIN;");
    // lock tables
    mysql_query( connect, "LOCK TABLES messages WRITE;" );
    unsigned long trID =( unsigned long ) mysql_thread_id( connect );
    cout << "MySQL Thread ID " << trID << endl;
    int err = mysql_query( connect, "SELECT id,email_to,send,resendtime,email_subject FROM messages WHERE send > 0 AND send < 100 AND resendtime < NOW() LIMIT 2;" );
    if( err ) {
        fprintf( stderr, "%s\n", mysql_error( connect ) );
        // mysql_close(con);
        // exit(1);
    }
    unsigned int i = 0;
    res_set = mysql_store_result( connect );
    // unsigned int numrows = mysql_num_rows(res_set);
    // int num_fields = mysql_num_fields(res_set);
    try {
        while((( row = mysql_fetch_row( res_set ) ) != NULL ) ) {
            cout << txt << " " << row[ i ] << " " << row[ i + 1 ] << " " << row[ i + 2 ] << " " << row[ i + 3 ] << " " << row[ i + 4 ] << endl;
           
            char query[ 500 ];
            sprintf( query, "UPDATE messages SET send=send+1,resendtime=DATE_ADD(NOW(), INTERVAL 5 MINUTE) WHERE id=%i;", atoi( row[ i ] ) );
            cout << "Query: " << query << endl;
            int err = mysql_query( connect, query );
            if( err ) {
                fprintf( stderr, "%s\n", mysql_error( connect ) );
                // mysql_close(con);
                // exit(1);
            }
            // add single row to vector string
            vector < string > msg;
            msg.push_back( row[ i ] );
            msg.push_back( row[ i + 1 ] );
            msg.push_back( row[ i + 2 ] );
            // add to messages
            messages.push_back( msg );
        }
    } catch( std::exception & e ) {
        cout << "Error pointer " << endl;
    }
    // lock tables end
    mysql_query( connect, "UNLOCK TABLES;" );
    //mysql_query (connect,"COMMIT;");
    mysql_free_result( res_set );
    mysql_close( connect );
    // return messages
    return messages;
}