Why would accessing this local member variable cause an exception? - c++

Question

I am trying to write a program that will read the contents of a .csv file into an array then print the results to the screen (part of a larger project) when the function getNumberOfRooms() is called an exception is thrown when it tries to return the value of the numberOfRooms variable, a private member within the class. Has anyone had a problem like this before or helped someone with this sort of problem? If so how did you solve it?
Thanks in advance,
Full source code available here: https://bitbucket.org/skutov/micropuzzle/
The exception that is thrown when getNumberOfRooms() is called:
Unhandled exception at 0x01354aa6 in MICROPUZZLE.exe: 0xC0000005: Access violation
reading location 0xccccccd0.
These are the functions in question (all of the time the variable is referenced in the class)
ClassMap::ClassMap ()
{
numberOfRooms = 0;
// Get number of rooms in map.csv
/* Find number of entries in map.csv file */
numberOfRooms = number_of_lines;
// allocate memory for rooms array
/* loading data from file into array */
}
}
// self explanitory
int ClassMap::getNumberOfRooms()
{
// Exception occurs on this line when accessing the variable
return numberOfRooms;
}
int ClassMap::printRoomDescriptions ()
{
for(int j = this->getNumberOfRooms(); j > 0; j--)
{
cout << roomArray[j].getDescription();
}
return 0;
}
Here is the class header:
class ClassMap
{
private:
int currentLocation;
int numberOfRooms;
// pointer to array initialised in constructor
ClassRoom *roomArray;
public:
// Constructors and Destructors
ClassMap();
~ClassMap();
// Print description, events and directions for current room
std::string getCurrentRoom();
// Change currentLocation to neighbour of current room if possible
int moveRoom(char direction);
// self explanitory
int getNumberOfRooms();
// dump room descriptions to command line (debugging)
int printRoomDescriptions();
};
Here is the constructor for ClassMap which also initializes roomArray:
ClassMap::ClassMap ()
{
numberOfRooms = 0;
// Get number of rooms in map.csv
unsigned int number_of_lines = 0;
FILE *infile = fopen("map.csv", "r");
int ch;
while (EOF != (ch=getc(infile)))
if ('\n' == ch)
++number_of_lines;
fclose(infile);
numberOfRooms = number_of_lines;
// allocate memory for rooms array
roomArray = new ClassRoom[numberOfRooms+1];
// set starting room
int currentLocation = 1;
// load that shit up
{
// Holders for values read from file
int newRoomID = 0;
char newRoomDescription[79] = "";
int newRoomNorthNeighbour = 0;
int newRoomEastNeighbour = 0;
int newRoomSouthNeighbour = 0;
int newRoomWestNeighbour = 0;
// used for iterations
int i = 0;
// File stream for map.csv
std::ifstream mapFile;
// Crack that shit open
mapFile.open ("map.csv");
// Line buffer for parsing
std::string line;
// For each line in the map.csv file read in the values into variables declared above then run initialise function for each room to store values into array
while (std::getline(mapFile, line))
{
// re-init parameters
newRoomID = 0;
newRoomNorthNeighbour = 0;
newRoomEastNeighbour = 0;
newRoomSouthNeighbour = 0;
newRoomWestNeighbour = 0;
for(i = 0;i<79;i++)
{
newRoomDescription[i] = ' ';
}
int parameter = 0;
int paraStart = 0;
int paraEnd = 0;
std::string buffer;
std::istringstream iss(line);
for(parameter = 0; parameter <= 5; parameter++)
{
// Empty buffer from last iteration
buffer.clear();
// Find end of current parameter
paraEnd = line.find(',',paraStart+1);
switch (parameter)
{
case 0:
buffer = line.substr((paraStart),(paraEnd-paraStart));
newRoomID = atoi(buffer.c_str());
break;
case 1:
buffer = line.substr((paraStart+2),(line.find("\"",paraStart+2)-(paraStart+2)));
for(i = 0;i<(buffer.length());i++)
{
newRoomDescription[i] = buffer.c_str()[i];
}
//newRoomDescription
break;
case 2:
buffer = line.substr((paraStart+1),(paraEnd-paraStart));
newRoomNorthNeighbour = atoi(buffer.c_str());
break;
case 3:
buffer = line.substr((paraStart+1),(paraEnd-paraStart));
newRoomEastNeighbour = atoi(buffer.c_str());
break;
case 4:
buffer = line.substr((paraStart+1),(paraEnd-paraStart));
newRoomSouthNeighbour = atoi(buffer.c_str());
break;
case 5:
buffer = line.substr((paraStart+1),(paraEnd-paraStart));
newRoomWestNeighbour = atoi(buffer.c_str());
break;
} // switch
// Cycle paraEnd to paraStart
paraStart = paraEnd;
} // for parameters loop
// Init next room with data
new (&roomArray[newRoomID]) ClassRoom( newRoomNorthNeighbour,
newRoomEastNeighbour,
newRoomSouthNeighbour,
newRoomWestNeighbour,
newRoomDescription);
} // while !EOF
// Close the file because we're a good little program and we don't need that shit no more
mapFile.close();
}
}

Answer 1

The key to this issue is:
Access violation reading location 0xccccccd0
0xcccccccc is a special value used in debug mode to denote an unitialised pointer. (See How to end up with a pointer to 0xCCCCCCCC ) It is set in debug mode to cause this kind of crash - it means the pointer you are using is not yet setup. Once you set up the pointer correctly the error will go away. (The slight difference from 0xcccccccc is the offset of the member you're trying to access inside that object.)
ADDED:
This is your error:
ClassRoom* roomArray = static_cast<ClassRoom*>( ::operator new ( sizeof ClassRoom * numberOfRooms ) );
This creates a local roomArray variable, and hides the member variable. What you really want is:
roomArray = static_cast<ClassRoom*>( ::operator new ( sizeof ClassRoom * numberOfRooms ) );
Or better yet:
roomArray = new ClassRoom[numberOfRooms];

Answer 2

I guess the problem is your for loop for(int j = this->getNumberOfRooms(); j > 0; j--). It should look like this: for(int j = this->getNumberOfRooms()-1; j >= 0; j--).
The last accessible index in an arry with N entries is N-1. On the other hand, the first index is 0.