Pure Virtual Function Call

Question

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... I'm rather pissed at my Professor for his wonderful 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; 
}

Answer 1

Well, I don't see the code that intializes the grid array. Perhaps grid it was created on the stack, or on the heap, and is therefore filled with uninitialized values. Uninitialized values could be anything, but they probably are not NULLs, and are definitely not valid pointers.

Answer 2

Not necessarily the cause of your problem (I haven't read your code too deeply), but you are not using virtual destructors, which you should be.

Answer 3

What is the definition of grid and how are you populating it? I bet you are doing it from the Animal constructor. At this time, the dynamic type of this is Animal, and not the type of the object that is eventually created.

Animal::Animal()
{
    grid[i][j] = this; // the type of this is Animal
}

Until the object is fully constructed, you cannot use the this pointer 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 this pointer, that is to say, storing it in the grid array, until after the object is fully constructed.

Here in the Animal constructor:

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); 
}

Note that you are calling Savanna::setAt with the this parameter. At this point, the dynamic type of this is Animal, not Wildebeest or some other thing. setAt does this:

void Savanna::setAt(int x, int y, Animal *anim) 
{ 
 if ((x>=0) && (x<SAVANNASIZE) && (y>=0) && (y<SAVANNASIZE)) 
 { 
  grid[x][y] = anim; 
 } 
}

The value of anim is 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:

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;
}

The WildeBeest named fred will go out of scope on the next line and be destroyed. You need to allocate it dynamically via new:

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 = new Wildebeest(&w, x, y);
                    placed = 1;
            }
    }
    placed = 0;
}

In the destrcuctor of the Savanna, there is a matching call to delete so that we do not leak memory:

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]); 
  } 
 } 
}

You will have exactly the same problem with the Lion instances as well.

Answer 4

The pure virtual error message means that the function being called doesn't have an implementation; it's effectively invoking a null pointer of type pointer to method. (That's why the syntax is =0;.) So, whatever else is going on, the error message is telling you there aren't any Wildebeasts being pointed to there.

I'd be real tempted to add a little code to check whether there's ANYTHING there. You've got a check for null, so the question is what is there?

It's almost certain that array isn't being initialized the right way.

Answer 5

I think your problem is not a call to pure virtual function (compiler can never allow that so it doesn't really cause a runtime error), but that you are calling the function on an invalid memory area that therefore has an invalid virtual table.

I am also willing to bet that this may have something to do with how you construct your beasts, as automatic variables rather than dynamically allocating them. As soon as you get out of those ifs, that memory is recycled.

Answer 6

One of your problems is these lines ...

   if (!(w.getAt(x, y))){
      Wildebeest fred(&w, x, y);
      placed = 1;
   }

... create a Wildebeest on the stack, and in the constructor the address of that stack-dwelling Wildebeest is stuffed into w's grid, and then the Wildebeest goes out of scope.

Your Wildebeests and Lions need to live in the heap ...

if (!(w.getAt(x, y))){

  // hey maintenance programmer, this looks like I'm leaking the Wildebeest,
  // but chillax, the Savannah is going to delete them 

  Wildebeest *fred = new Wildebeest(&w, x, y);
  placed = 1;

}

... and you need the comment because what you're doing is far, far away from idiomatic C++.