delete array pointers heap corruption - c++

Question

I get an exception on this line in Visual Studio 2015. It builds with no errors.
_free_dbg(block, _UNKNOWN_BLOCK);
This is how I declare the new array of pointers:
CAirship * pAirShip[10];
This is how I delete the array of pAirShip pointers:
for (int i = 0; i < 10; i++) {
if (pAirShip[i]) {
cout << "pAirShip[" << i << "] is " << pAirShip[i] << endl;
delete pAirShip[i];// Delete appropriate object
}
} // end for loop
I get an error on attempting to delete pAirShip[0],
Here is a debug window that does print the pointer addresses:
Here is the full code:
struct AirShipFile {
int Type; // Airplane or Balloon
string name; // Name of the airship
int passCount; // passenger count
int weightCargo; // cargo weight
int EngOrGas; // engine or gas type
int distance; // range or altitude
};
enum EngineType { Jet, Propeller }; // for airplanes only
std::ostream& operator<<(std::ostream& out, const EngineType value) {
static std::map<EngineType, std::string> strings;
if (strings.size() == 0) {
#define INSERT_ELEMENT(p) strings[p] = #p
INSERT_ELEMENT(Jet);
INSERT_ELEMENT(Propeller);
#undef INSERT_ELEMENT
}
return out << strings[value];
}
enum GasType {Helium, Hydrogen }; // for proprellers only
std::ostream& operator<<(std::ostream& out, const GasType value) {
static std::map<GasType, std::string> strings;
if (strings.size() == 0) {
#define INSERT_ELEMENT(p) strings[p] = #p
INSERT_ELEMENT(Helium);
INSERT_ELEMENT(Hydrogen);
#undef INSERT_ELEMENT
}
return out << strings[value];
}
enum AirShipType { AIRPLANE, BALLOON };
class CAirship {
public:
CAirship() { }
virtual void SetData(AirShipFile &data) = 0;
virtual void GetData() = 0;
AirShipType GetAirShipType() { return m_AirShipType; }
protected:
AirShipType m_AirShipType;
};
class CAIRPLANE : public virtual CAirship {
public:
CAIRPLANE() : CAirship() {}
void SetData(AirShipFile &data);
void GetData();
private:
EngineType m_EngineType;
int m_MaxPassengerCount;
string m_Name;
int m_MaxCargoWeight;
int m_MaxAltitude;
};
// Function: SetData
void CAIRPLANE::SetData(AirShipFile &data)
{
// cast integer to enum
m_EngineType = EngineType(data.EngOrGas);
// airplane name
m_Name = data.name;
// passenger count
m_MaxPassengerCount = data.passCount;
//max cargo weight
m_MaxCargoWeight = data.weightCargo;
// cast integer to enum
m_AirShipType = AirShipType(data.Type);
// maximum altitude
m_MaxAltitude = data.distance;
}
void CAIRPLANE::GetData()
{
cout << setw(20) << m_Name << "\t" << setw(20) << m_EngineType << setw(20);
cout << left << setw(20) << m_MaxAltitude << "\n";
}
class CBALLOON : public virtual CAirship {
public:
CBALLOON() : CAirship() {}
void SetData(AirShipFile &data);
void GetData();
private:
GasType m_GasType;
EngineType m_EngineType;
int m_MaxPassengerCount;
string m_Name ;
int m_MaxCargoWeight;
int m_MaxAltitude;
};
void CBALLOON::SetData(AirShipFile &data)
{
// cast integer to enum
m_GasType = GasType(data.EngOrGas);
// airplane name
m_Name = data.name;
// passenger count
m_MaxPassengerCount = data.passCount;
//max cargo weight
m_MaxCargoWeight = data.weightCargo;
// cast integer to enum
m_AirShipType = AirShipType(data.Type);
// maximum altitude
m_MaxAltitude = data.distance;
}
void CBALLOON::GetData()
{
cout << setw(20) << m_Name << "\t" << setw(20)<< m_GasType << setw(20);
cout << left << setw(20) << m_MaxAltitude << "\n";
}
// AIRPLANE = 0
// BALLOON = 1
int main(int argc, char *argv[])
{
if (argc != 2) {
cout << "Usage: PR <filename>\n";
return 1;
}
ifstream Infile(argv[1]);
if (!Infile) {
cout << "Cannot open file\n";
return 1;
}
char LineBuf[100];
char d[] = ",";
CAirship * pAirShip[10];
int i = 0;
while (Infile.getline(LineBuf, 100)) {
struct AirShipFile data;
// read the first field Airship type
// airplane or balloon
data.Type = atoi(strtok(LineBuf, d));
switch (data.Type) {
case AIRPLANE:
// Create AIRPLANE Object
pAirShip[i] = new CAIRPLANE();
data.name = strtok(NULL, d);
data.passCount = atoi(strtok(NULL, d));
data.weightCargo = atoi(strtok(NULL, d));
data.EngOrGas = atoi(strtok(NULL, d));
data.distance = atoi(strtok(NULL, d));
break;
case BALLOON:
// Create BALLOON Object
pAirShip[i] = new CBALLOON();
data.name = strtok(NULL, d);
data.passCount = atoi(strtok(NULL, d));
data.weightCargo = atoi(strtok(NULL, d));
data.EngOrGas = atoi(strtok(NULL, d));
data.distance = atoi(strtok(NULL, d));
break;
default:
break;
} // end switch
// call appropriate function
pAirShip[i++]->SetData(data);
memset(LineBuf, '\0', 100);
}
Infile.close();
cout << "Listing of all Airplanes \n";
cout << left << setw(20) << "\nName" << left<< setw(20)<<"\tEngine Type";
cout << left<<setw(20)<<"\Maximum Range" << "\n\n";
for (int i = 0; i < 10; i++) {
if (pAirShip[i]->GetAirShipType() == AIRPLANE)
pAirShip[i]->GetData();
}
cout << "\n\nListing of all Balloons \n";
cout <<left << setw(20) << "\nName" << left << setw(20) << "\tGas Type" ;
cout << left << setw(20) << "\Maximum Altitude" << "\n\n";
for (int i = 0; i < 10; i++) {
if (pAirShip[i]->GetAirShipType() == BALLOON)
pAirShip[i]->GetData();
}
for (int i = 0; i < 10; i++) {
if (pAirShip[i]) {
delete pAirShip[i];// Delete appropriate object
}
} // end for loop
return 0;
}

Answer 1

The problem is that when allocating an array of any kind, C++ does not initialize the elements, but leaves them with "random" values. So, when you create an array of pointers, the pointers are not created with NULL, nullptr or 0 value, so this is not a good indicator if they are really unused on its own. Trying to free the space that isn't allocated is what generates the error. You should first initialize them (by yourself in a for loop) with nullptr right after you create the array, then you can use your code for deleting the array of pointers.