I obviously do not ‘grok’ C++.
On this programming assignment, I have hit a dead end. A runtime error occurs at this line of code:
else if (grid[i][j]->getType() == WILDEBEEST) { ...
with the message “Runtime Error – pure virtual function call.”
From my understanding, this error occurs if the function reference attempts to call the (virtual) base class while the child class is not currently instantiated. However, I do not see where I have made this mistake.
Relevant Code:
Professor’s Code:
const int LION = 1;
const int WILDEBEEST = 2;
//
// .
// .
// .
//
class Animal {
friend class Savanna; // Allow savanna to affect animal
public:
Animal();
Animal(Savanna *, int, int);
~Animal();
virtual void breed() = 0; // Breeding implementation
virtual void move() = 0; // Move the animal, with appropriate behavior
virtual int getType() = 0; // Return if wildebeest or lion
virtual bool starve() = 0; // Determine if animal starves
protected:
int x,y; // Position in the savanna, using the XY coordinate plane
bool moved; // Bool to indicate if moved this turn
int breedTicks; // Number of ticks since breeding
Savanna *savanna;
};
//
// .
// .
// .
//
void Savanna::Display()
{
int i,j;
cout << endl << endl;
for (j=0; j<SAVANNASIZE; j++)
{
for (i=0; i<SAVANNASIZE; i++)
{
if (grid[i][j]==NULL){
setrgb(0);
cout << " ";
}
else if (grid[i][j]->getType()==WILDEBEEST) // RUNTIME ERROR HERE
{
setrgb(7);
cout << "W";
}
else {
setrgb(3);
cout << "L";
}
}
cout << endl;
}
setrgb(0);
}
My Code:
class Wildebeest: public Animal {
friend class Savanna; // Allow the Savanna to affect the animal, as per spec
public:
Wildebeest();
Wildebeest(Savanna *, int, int); // accepts (pointer to a Savanna instance, X Position, Y Position)
void breed(); // Perform breeding, and check breedTick
void move(); // move the animal.
int getType(); // returns WILDEBEEST
bool starve(); // if starving, returns 0. (counterintuitive, I know.)
};
int Wildebeest::getType() {
return WILDEBEEST;
}
I’ve read The Old New Thing: What is __purecall? and Description of the R6025 run-time error in Visual C++ but I do not fully understand why this is occurring in the above code.
[edit] Full listing of main.c (yes, all one file… part of the assignment requirements.)
//
// This program simulates a 2D world with predators and prey.
// The predators (lions) and prey (wildebeest) inherit from the
// Animal class that keeps track of basic information about each
// animal (time ticks since last bred, position on the savanna).
//
// The 2D world is implemented as a separate class, Savanna,
// that contains a 2D array of pointers to type Animal.
//
// ****************************************************************
#include <iostream>
#include <string>
#include <vector>
#include <cstdlib>
#include <time.h>
#include "graphics.h"
using namespace std;
int wrapTo20 (int value) {
if (0 > value) {
value = 19;
} else if (20 == value) {
value = 0;
}
return value;
}
const int SAVANNASIZE = 20;
const int INITIALBEEST = 100;
const int INITIALLIONS = 5;
const int LION = 1;
const int WILDEBEEST = 2;
const int BEESTBREED = 3;
const int LIONBREED = 8;
const int LIONSTARVE = 3;
// Forward declaration of Animal classes so we can reference it
// in the Savanna class
class Animal;
class Lion;
class Wildebeest;
// ==========================================
// The Savana class stores data about the savanna by creating a
// SAVANNASIZE by SAVANNASIZE array of type Animal.
// NULL indicates an empty spot, otherwise a valid object
// indicates an wildebeest or lion. To determine which,
// invoke the virtual function getType of Animal that should return
// WILDEBEEST if the class is of type Wildebeest, and Lion otherwise.
// ==========================================
class Savanna
{
friend class Animal; // Allow Animal to access grid
friend class Lion; // Allow Animal to access grid
friend class Wildebeest; // Allow Animal to access grid
public:
Savanna();
~Savanna();
Animal* getAt(int, int);
void setAt(int, int, Animal *);
void Display();
void SimulateOneStep();
private:
Animal* grid[SAVANNASIZE][SAVANNASIZE];
};
// ==========================================
// Definition for the Animal base class.
// Each animal has a reference back to
// the Savanna object so it can move itself
// about in the savanna.
// ==========================================
class Animal
{
friend class Savanna; // Allow savanna to affect animal
public:
Animal();
Animal(Savanna *, int, int);
~Animal();
virtual void breed() = 0; // Whether or not to breed
virtual void move() = 0; // Rules to move the animal
virtual int getType() = 0; // Return if wildebeest or lion
virtual bool starve() = 0; // Determine if animal starves
protected:
int x,y; // Position in the savanna
bool moved; // Bool to indicate if moved this turn
int breedTicks; // Number of ticks since breeding
Savanna *savanna;
};
// ======================
// Savanna constructor, destructor
// These classes initialize the array and
// releases any classes created when destroyed.
// ======================
Savanna::Savanna()
{
// Initialize savanna to empty spaces
int i,j;
for (i=0; i<SAVANNASIZE; i++)
{
for (j=0; j<SAVANNASIZE; j++)
{
grid[i][j]=NULL;
}
}
}
Savanna::~Savanna()
{
// Release any allocated memory
int i,j;
for (i=0; i<SAVANNASIZE; i++)
{
for (j=0; j<SAVANNASIZE; j++)
{
if (grid[i][j]!=NULL) delete (grid[i][j]);
}
}
}
// ======================
// getAt
// Returns the entry stored in the grid array at x,y
// ======================
Animal* Savanna::getAt(int x, int y)
{
if ((x>=0) && (x<SAVANNASIZE) && (y>=0) && (y<SAVANNASIZE))
return grid[x][y];
return NULL;
}
// ======================
// setAt
// Sets the entry at x,y to the
// value passed in. Assumes that
// someone else is keeping track of
// references in case we overwrite something
// that is not NULL (so we don't have a memory leak)
// ======================
void Savanna::setAt(int x, int y, Animal *anim)
{
if ((x>=0) && (x<SAVANNASIZE) && (y>=0) && (y<SAVANNASIZE))
{
grid[x][y] = anim;
}
}
// ======================
// Display
// Displays the savanna in ASCII. Uses W for wildebeest, L for lion.
// ======================
void Savanna::Display()
{
int i,j;
cout << endl << endl;
for (j=0; j<SAVANNASIZE; j++)
{
for (i=0; i<SAVANNASIZE; i++)
{
if (grid[i][j]==NULL){
setrgb(0);
cout << " ";
}
else if (grid[i][j]->getType()==WILDEBEEST)
{
setrgb(7);
cout << "W";
}
else {
setrgb(3);
cout << "L";
}
}
cout << endl;
}
setrgb(0);
}
// ======================
// SimulateOneStep
// This is the main routine that simulates one turn in the savanna.
// First, a flag for each animal is used to indicate if it has moved.
// This is because we iterate through the grid starting from the top
// looking for an animal to move . If one moves down, we don't want
// to move it again when we reach it.
// First move lions, then wildebeest, and if they are still alive then
// we breed them.
// ======================
void Savanna::SimulateOneStep()
{
int i,j;
// First reset all animals to not moved
for (i=0; i<SAVANNASIZE; i++)
for (j=0; j<SAVANNASIZE; j++)
{
if (grid[i][j]!=NULL) grid[i][j]->moved = false;
}
// Loop through cells in order and move if it's a Lion
for (i=0; i<SAVANNASIZE; i++)
for (j=0; j<SAVANNASIZE; j++)
{
if ((grid[i][j]!=NULL) && (grid[i][j]->getType()==LION))
{
if (grid[i][j]->moved == false)
{
grid[i][j]->moved = true; // Mark as moved
grid[i][j]->move();
}
}
}
// Loop through cells in order and move if it's an Wildebeest
for (i=0; i<SAVANNASIZE; i++)
for (j=0; j<SAVANNASIZE; j++)
{
if ((grid[i][j]!=NULL) && (grid[i][j]->getType()==WILDEBEEST))
{
if (grid[i][j]->moved == false)
{
grid[i][j]->moved = true; // Mark as moved
grid[i][j]->move();
}
}
}
// Loop through cells in order and check if we should breed
for (i=0; i<SAVANNASIZE; i++)
for (j=0; j<SAVANNASIZE; j++)
{
// Kill off any lions that haven't eaten recently
if ((grid[i][j]!=NULL) &&
(grid[i][j]->getType()==LION))
{
if (grid[i][j]->starve())
{
delete (grid[i][j]);
grid[i][j] = NULL;
}
}
}
// Loop through cells in order and check if we should breed
for (i=0; i<SAVANNASIZE; i++)
for (j=0; j<SAVANNASIZE; j++)
{
// Only breed animals that have moved, since
// breeding places new animals on the map we
// don't want to try and breed those
if ((grid[i][j]!=NULL) && (grid[i][j]->moved==true))
{
grid[i][j]->breed();
}
}
}
// ======================
// Animal Constructor
// Sets a reference back to the Savanna object.
// ======================
Animal::Animal()
{
savanna = NULL;
moved = false;
breedTicks = 0;
x=0;
y=0;
}
Animal::Animal(Savanna *savana, int x, int y)
{
this->savanna = savana;
moved = false;
breedTicks = 0;
this->x=x;
this->y=y;
savanna->setAt(x,y,this);
}
// ======================
// Animal destructor
// No need to delete the savanna reference,
// it will be destroyed elsewhere.
// ======================
Animal::~Animal()
{ }
// Start with the Wildebeest class and its required declarations
class Wildebeest: public Animal {
friend class Savanna; // Allow savanna to affect animal
public:
Wildebeest();
Wildebeest(Savanna *, int, int);
void breed(); // Whether or not to breed
void move(); // Rules to move the animal
int getType(); // Return if wildebeest or lion
bool starve();
};
bool Wildebeest::starve() {
return 1;
}
// ======================
// Wildebeest constructors
// ======================
Wildebeest::Wildebeest() {
}
Wildebeest::Wildebeest(Savanna * sav, int x, int y) : Animal(sav, x, y) {
}
// ======================
// Wldebeest Move
// Look for an empty cell up, right, left, or down and
// try to move there.
// ======================
void Wildebeest::move() {
int loc1, loc2, loc3, loc4;
int x1, x2, x3, x4;
int y1, y2, y3, y4;
x1 = wrapTo20(x);
y1 = wrapTo20(y + 1);
x2 = wrapTo20(x + 1);
y2 = wrapTo20(y);
x3 = wrapTo20(x);
y3 = wrapTo20(y - 1);
x4 = wrapTo20(x - 1);
y4 = wrapTo20(y);
loc1 = savanna->getAt(x1, y1)->getType();
loc2 = (int)savanna->getAt(x2, y2)->getType();
loc3 = savanna->getAt(x3, y3)->getType();
loc4 = savanna->getAt(x4, y4)->getType();
while (!moved) {
int x = 1 + (rand() % 4);
switch (x) {
case 1:
if (!loc1) savanna->setAt(x1, y1, this);
break;
case 2:
if (!loc2) savanna->setAt(x2, y2, this);
break;
case 3:
if (!loc3) savanna->setAt(x3, y3, this);
break;
case 4:
if (!loc4) savanna->setAt(x4, y4, this);
break;
default:
break;
}
}
}
// ======================
// Wildebeest getType
// This virtual function is used so we can determine
// what type of animal we are dealing with.
// ======================
int Wildebeest::getType() {
return WILDEBEEST;
}
// ======================
// Wildebeest breed
// Increment the tick count for breeding.
// If it equals our threshold, then clone this wildebeest either
// above, right, left, or below the current one.
// ======================
void Wildebeest::breed() {
breedTicks++;
if (2 == breedTicks) {
breedTicks = 0;
}
}
// *****************************************************
// Now define Lion Class and its required declarations
// *****************************************************
class Lion: public Animal {
friend class Savanna; // Allow savanna to affect animal
public:
Lion();
Lion(Savanna *, int, int);
void breed(); // Whether or not to breed
void move(); // Rules to move the animal
int getType(); // Return if wildebeest or lion
bool starve();
};
// ======================
// Lion constructors
// ======================
Lion::Lion() {
}
Lion::Lion(Savanna * sav, int x, int y) : Animal(sav, x, y) {
}
// ======================
// Lion move
// Look up, down, left or right for a lion. If one is found, move there
// and eat it, resetting the starveTicks counter.
// ======================
void Lion::move() {
int loc1, loc2, loc3, loc4;
int x1, x2, x3, x4;
int y1, y2, y3, y4;
x1 = wrapTo20(x);
y1 = wrapTo20(y + 1);
x2 = wrapTo20(x + 1);
y2 = wrapTo20(y);
x3 = wrapTo20(x);
y3 = wrapTo20(y - 1);
x4 = wrapTo20(x - 1);
y4 = wrapTo20(y);
loc1 = savanna->getAt(x1, y1)->getType();
loc2 = (int)savanna->getAt(x2, y2)->getType();
loc3 = savanna->getAt(x3, y3)->getType();
loc4 = savanna->getAt(x4, y4)->getType();
while (!moved) {
int x = 1 + (rand() % 4);
switch (x) {
case 1:
if (!loc1) savanna->setAt(x1, y1, this);
break;
case 2:
if (!loc2) savanna->setAt(x2, y2, this);
break;
case 3:
if (!loc3) savanna->setAt(x3, y3, this);
break;
case 4:
if (!loc4) savanna->setAt(x4, y4, this);
break;
default:
break;
}
}
}
// ======================
// Lion getType
// This virtual function is used so we can determine
// what type of animal we are dealing with.
// ======================
int Lion::getType() {
return LION;
}
// ======================
// Lion breed
// Creates a new lion adjacent to the current cell
// if the breedTicks meets the threshold.
// ======================
void Lion::breed() {
breedTicks++;
if (2 == breedTicks) {
breedTicks = 0;
}
}
// ======================
// Lion starve
// Returns true or false if a lion should die off
// because it hasn't eaten enough food.
// ======================
bool Lion::starve() {
return 1;
}
// ======================
// main function
// ======================
int main()
{
string s;
srand((int)time(NULL)); // Seed random number generator
Savanna w;
int initialWildebeest=0;
int initialLions=0;
// enter initial number of wildebeest
int beestcount = 0;
while(initialWildebeest <= 0 || initialWildebeest > INITIALBEEST){
cout << "Enter number of initial Wildebeest (greater than 0 and less than " << INITIALBEEST << ") : ";
cin >> initialWildebeest;
}
// Randomly create wildebeests and place them in a randomly choosen empty spot in savanna
int i;
bool placed = 0;
for ( i = 0; i < initialWildebeest; i++) {
while (!placed) {
int x = 1 + (rand() % 20);
int y = 1 + (rand() % 20);
if (!(w.getAt(x, y))){
Wildebeest fred(&w, x, y);
placed = 1;
}
}
placed = 0;
}
// Enter initial number of lions
int lioncount = 0;
while(initialLions <= 0 || initialLions > INITIALLIONS){
cout << "Enter number of initial Lions (greater than 0 and less than " << INITIALLIONS << ") : ";
cin >> initialLions;
}
// Randomly create lions and place them in a randomly choosen empty spot in savanna
placed = 0;
for ( i = 0; i < initialLions; i++) {
while (!placed) {
int x = 1 + (rand() % 20);
int y = 1 + (rand() % 20);
if (!(w.getAt(x, y))){
Lion ronald(&w, x, y);
placed = 1;
}
}
placed = 0;
}
// Run simulation forever, until user cancels
int count=0;
while (true)
{
gotoxy(0,0);
w.Display();
w.SimulateOneStep();
Sleep(500);
count++;
if(count == 20){
count=0;
cout << endl << "Press enter for next step, ctrl-c to quit" << endl;
getline(cin,s);
clearline(23);
}
}
return 0;
}
What is the definition of
gridand how are you populating it? I bet you are doing it from the Animal constructor. At this time, the dynamic type of this isAnimal, and not the type of the object that is eventually created.Until the object is fully constructed, you cannot use the
thispointer in a dynamic way, this includes calling virtual functions, or using the virtual function table.To be more specific, you need to postpone using the
thispointer, that is to say, storing it in thegridarray, until after the object is fully constructed.Here in the Animal constructor:
Note that you are calling
Savanna::setAtwith thethisparameter. At this point, the dynamic type ofthisis Animal, not Wildebeest or some other thing. setAt does this:The value of
animis the this pointer from the Animal constructor.Note a few more things. When you are constructing the list of Wildebeest, you are causing a dangling pointer:
The WildeBeest named
fredwill go out of scope on the next line and be destroyed. You need to allocate it dynamically vianew:In the destrcuctor of the Savanna, there is a matching call to delete so that we do not leak memory:
You will have exactly the same problem with the Lion instances as well.