Debug Assertion Failed: Invalid Null Pointer - c++

Question

I am attempting to make a test program to see if an idea I had for getting and storing data from an SQLite3 database in a struct would work but I am running into some major problems with the code. While debugging I keep running into the error in the title. Here is the full text of the error window:
Debug Assertion Failed!
File: include\xstring
Line: 929
Expression: invalid null pointer
For information on how your program can cause an assertion
failure, see the Visual C++ documentation on asserts.
And here is the code, I will indicate which line the problem is with an arrow (<--; extra help see case 2 of the switch):
#include <iostream>
#include "data_placeholder.h"
#include "sqlite3.h"
#include <vector>
#include <conio.h>
#include <string>
#include <sstream>
using namespace std;
void openDB(sqlite3* dBase, int iID, string table, string operation, sqlite3_stmt* statement, vector<mission>& mission_1);
void createStatement(int iID, string table, string operation, sqlite3_stmt* statement, sqlite3* dBase, vector<mission>& mission_1);
void getMResults(string sqlStr, sqlite3_stmt* statement, sqlite3* dBase, vector<mission>& mission_1);
void returnMResult(vector<mission> mResults, vector<mission>& mission_1);
int main()
{
//Define Variables
vector<mission> mission_1;
sqlite3 *dBase;
sqlite3_stmt *statement;
int pInput;
mission_1.push_back(mission());
cout << "Input a number between 1 and 3" << endl;
cout << ">";
cin >> pInput;
cout << endl;
cout << endl;
openDB(dBase, pInput, "Mission_Data", "select from", statement, mission_1);
cout << mission_1.at(0).mName << ", " << mission_1.at(0).mDesc << ", " << mission_1.at(0).mCBELevel << ", " << mission_1.at(0).mSCReq << ", " << mission_1.at(0).mMWReq << ", " << mission_1.at(0).mTLimit << ", " << mission_1.at(0).mDifficulty << ", " << mission_1.at(0).mSector << ", " << mission_1.at(0).mSystem << ", " << mission_1.at(0).mTName << ", " << mission_1.at(0).mTSClass << ", " << mission_1.at(0).mBounty << ", " << mission_1.at(0).mXarn << ", " << mission_1.at(0).mRubies << ", " << mission_1.at(0).mDiamonds << ", " << mission_1.at(0).mDraconic << ", " << mission_1.at(0).mLithium << ", " << mission_1.at(0).mPlatinum << ", " << mission_1.at(0).mUranium << ", " << mission_1.at(0).mPlutonium << ", " << mission_1.at(0).mNWaste << ", " << mission_1.at(0).mCEXP << ", " << mission_1.at(0).mItem << ", " << mission_1.at(0).mType << ", " << endl;
_getch();
}
void openDB(sqlite3* dBase, int iID, string table, string operation, sqlite3_stmt* statement, vector<mission>& mission_1)
{
sqlite3_open("scDatabase.sqlite",&dBase);
createStatement(iID, table, operation, statement, dBase, mission_1);
}
void createStatement(int iID, string table, string operation, sqlite3_stmt* statement, sqlite3* dBase, vector<mission>& mission_1)
{
stringstream ss;
ss << iID;
string sID(ss.str());
string sqlStr = "Select * From " + table + " Where ID = " + sID;
getMResults(sqlStr, statement, dBase, mission_1);
}
void getMResults(string sqlStr, sqlite3_stmt* statement, sqlite3* dBase, vector<mission>& mission_1)
{
vector<mission> mResults;
mResults.push_back(mission());
if (sqlite3_prepare_v2(dBase, sqlStr.c_str(), sqlStr.size(), &statement, 0) == SQLITE_OK)
{
int cols;
int i;
cols = sqlite3_column_count(statement);
for (i =01; i <= cols; i++)
{
switch(i)
{
case 2:
mResults.at(0).mName = string((char*)sqlite3_column_text(statement,i)); //<-- Here is the line the assert fail happens
break;
/*
case 3:
mResults.at(0).mDesc = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 4:
mResults.at(0).mCBELevel = sqlite3_column_int(statement,i);
break;
case 5:
mResults.at(0).mSCReq = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 6:
mResults.at(0).mMWReq = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 7:
mResults.at(0).mTLimit = sqlite3_column_int(statement,i);
break;
case 8:
mResults.at(0).mDifficulty = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 9:
mResults.at(0).mSector = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 10:
mResults.at(0).mSystem = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 11:
mResults.at(0).mTName = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 12:
mResults.at(0).mTSClass = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
case 13:
mResults.at(0).mBounty = sqlite3_column_int(statement,i);
break;
case 14:
mResults.at(0).mXarn = sqlite3_column_int(statement,i);
break;
case 15:
mResults.at(0).mRubies = sqlite3_column_int(statement,i);
break;
case 16:
mResults.at(0).mDiamonds = sqlite3_column_int(statement,i);
break;
case 17:
mResults.at(0).mDraconic = sqlite3_column_int(statement,i);
break;
case 18:
mResults.at(0).mLithium = sqlite3_column_int(statement,i);
break;
case 19:
mResults.at(0).mPlatinum = sqlite3_column_int(statement,i);
break;
case 20:
mResults.at(0).mNWaste = sqlite3_column_int(statement,i);
break;
case 21:
mResults.at(0).mCEXP = sqlite3_column_int(statement,i);
break;
case 22:
mResults.at(0).mItem = sqlite3_column_int(statement,i);
break;
case 23:
mResults.at(0).mType = string(reinterpret_cast<const char*>(sqlite3_column_text(statement,i)));
break;
*/
default:
break;
}
}
}
else
{
cout << "something is wrong" << endl;
}
returnMResult(mResults, mission_1);
}
void returnMResult(vector<mission>mResults, vector<mission>& mission_1)
{
mission_1.at(0) = mResults.at(0);
}
The error occurs instantly when the code hits this line on the first iteration through the for loop. There are no compiler errors and I have also tried collapsing the first three functions into one function in case the database and statement pointers were not being passed correctly; same problem.
Edit 2: I have whittled down where the problem is. It has to do with my vector of structs. I took the database query out of the link where I set to mResults.at(0).mName and then added a cast for the const unsigned char to string but the assert failure still happens.
Edit 3: After looking at some code I had done earlier in the the year I have figure out what was going on, at least for the SQLite query. You have to called step in order for the query to actually be carried out. Since I had not done so the pointer was always returning as invalid since there was no row loaded and thus no columns to query. Nothing wrong with my vector.

Answer 1

problematic line:
sqlite3_column_text(statement,i)
will return undefined value, because when i is equal to size, it will go out of bounds.
sqlite3_column_text function iCol parameter is C-style index, that starts with zero, while you try to get column index sql-style starting 1. Fix the loop to be:
for (i = 0; i < cols; i++)

Answer 2

I figured out the problem. I looked back at some code I had made around June that I had working and started comparing it to the code I posted above. In the places where there were differences I copied the code over to the new test and I finally got it to work.
The problem was that I was not calling sqlite3_step so the database was not being queried. This lead to no row being loaded and so no columns to query, thus an invalid pointer returned by the sqlite3_column_text. However Iuri also had a point in that I was iterating through in a way that, if the error I was getting was fixed, would have started going out of bounds, I had not been able to get the code to go that far yet so a bit of preemptive debugging.
I also added checks in for some basic defensive coding so that the test application fails save if there is either not a row loaded or an invalid pointer so that the runtime does not kick the program out.