operatoroverloading - []

hello i get debug error when i going to overload [] operator..
i compile the program in VC++ 6.0 and get error DEMAGE: aftr normal
block(#51)..
my program get run and also get output but at the end of clossing
brecket i got this error...

My Program:
include <iostream.h>
#include <iomanip.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <conio.h>

#define LIST 10
class array
{
    char *str[LIST];
    int a[LIST];
    int b;

public:
    array()
    {
        for(int i=0;i<LIST;i++)
        {
            str[i]=NULL;
            a[i]=0;
        }

        b=0;
    }
    ~array()
    {
        for(int i=0;i<b;i++)
            delete str[i];
    }
    int & operator [] (char * st)
    {
            b++;
            for(int i=0;i<b;i++)
            {
                if(str[i]!=NULL)
                    if(strcmp(str[i],st)==0)return a[i];

            }

            if((str[b] = (char *)malloc(sizeof(st)))==NULL)
                cout<<"allocation error\n";
            strcpy(str[b],st);
            return a[b];

    }

};

main()
{
    array a;
    int b;
    a["dharmesh"] = 10;
    a["manish"] ;
    a["dha"]=30;

    b=a["dha"];

    printf("%d",b);
    b=a["dharmesh"];
    printf("%d",b);

}