c++ - overloading array subscript operator

Question

I'm trying to provide custom exception handling mechanism to a class inherited from mysqlpp::StoreQueryResult in mysql++ library. But what i'm finding really hard to do is to find a way to refer to the actual object within the class, so i'm unable to use the vector operation at() properly in order to retrieve a value at an index.

Here is the header

/* MySQLQueryResult.h */


#ifndef MYSQLQUERYRESULT_H
#define MYSQLQUERYRESULT_H

#include <mysql++.h>
#include <result.h>

namespace  MinesScanner {

    namespace MoonStone {

        class MySQLQueryResult : public mysqlpp::StoreQueryResult  {
        public:

            MySQLQueryResult();


            MySQLQueryResult(const MySQLQueryResult &other);

            MySQLQueryResult& operator=(const MySQLQueryResult &other);

            mysqlpp::Row& operator[](int index);

        private:

            int _dat;



        };

    }

}

#endif  /* MYSQLQUERYRESULT_H */

Here is the source file

/* MySQLQueryResult.cpp */

#include "MySQLQueryResult.h"

namespace MinesScanner {

    namespace MoonStone {

        MySQLQueryResult::MySQLQueryResult( )
    : StoreQueryResult( )
    {
    }

    MySQLQueryResult::MySQLQueryResult( const StoreQueryResult &ob )
    : StoreQueryResult( ob )
    {
    }

    MySQLQueryResult& MySQLQueryResult::operator=( const StoreQueryResult &ob )
    {

        StoreQueryResult::operator=( ob ) ;
        return *this ;

    }

    mysqlpp::Row& MySQLQueryResult::operator[]( int index )
    {


        try {

            std::cout << " Called " << this->at( index ) << std::endl ;
            return this->at( index ) ;
        } catch ( std::exception& excpn_ob ) {
            std::cerr << " Exception caught : " << excpn_ob.what( ) << std::endl ;
        }


    }


    }

}

A simple usage example will show more clearly what i want to achieve.

#include "MySQLQueryResult.h"

int main() {
StoreQueryResult lisres = getMinesData( ( string ) row.at( 0 ) ) ; // returns StoreQueryResult object storing the result

cout << lisres[0][7] << endl; // outputs "Falkreath Stormcloak Mines"

MySQLQueryResult my_lisres = getMinesData( ( string ) row.at( 0 ) ) ; // returns StoreQueryResult object storing the result

cout << my_lisres[0][7] << endl; // ERROR!

}

So i basically want to add more boundary checking, check for null values , and handle out_of_range exception using the operator[] in class MySQLQueryResult but its not working.

I want to be able to access MySQLQueryResult object using array subscripts . Either i'm getting a garbage value or a Seg fault. Please let me know how to get this right

Has your program written output message which you define inside subscript operator? — Gluttton
– Gluttton, Commented Jun 28, 2014 at 20:18
Yes its printing "Called" for every usage of the array subscript operator — Maxx
– Maxx, Commented Jun 29, 2014 at 6:24

Gluttton · Accepted Answer · 2014-06-28 20:09:09Z

3

but its not working

It looks like you also need to overload const version of subscript operator:

const mysqlpp::Row& operator[](int index) const;

edited Jun 28, 2014 at 20:09

answered Jun 28, 2014 at 19:34

Gluttton

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