C++ Fixing a programming excercise - c++

Question

My teacher asked the class to fix the error in this program. Actually it seems to be a crappy program; I just typed it exactly how it is in the sheet, and got this error:
Well Now I have just change some things, but get this exception at run time: Microsoft C++ exception: [rethrow] at memory location 0x00000000..
The code is now like so: (The variable an class names are now in spanish, sorry for the inconviniences)
#include <iostream>
#include <exception>
#include <stack>
using namespace std;
class EPilaVacia : public exception{
public:
const char* what() const throw(){
return "Error: Pila Vacía";
}
};
template <class T, int max=100>
class Pila{
private:
stack<T*> *pila;
int cont;
public:
Pila() : cont(0){
pila=new stack<T*>();
}
virtual void apilar( T* pt){
if(cont<max){
pila->push(pt); //respuesta 2
}
}
virtual void apilar(T t){
if(cont<max){
pila->push(&t); //respuesta 3
}
}
T tope() const throw (EPilaVacia){
if(cont>0){
pila->top(); //respuesta 4
}else{
throw ; //respuesta 5
}
}
T& desapilar() throw (EPilaVacia){
if(cont>0){
pila->pop(); //respuesta 6
}else{
throw ; //respuesta 7
}
}
int size() const{
return pila->size();
}
};
class Figura{
public:
virtual void print(){
cout<< "FIGURA" <<endl;
}
};
class Circulo : public Figura{
public:
void print(){
cout<<"CIRCULO"<<endl;
}
};
class Triangulo : public Figura{
public:
void print(){
cout<<"TRIANGULO"<<endl;
}
};
int main(){
Pila<Figura*> *pfiguras= new Pila<Figura*>();
pfiguras->apilar(new Circulo());
pfiguras->apilar(new Triangulo());
Pila<Figura*> pfiguras2(*pfiguras);
pfiguras->tope()->print();
pfiguras->desapilar();
pfiguras->tope()->print();
pfiguras->desapilar();
pfiguras2.tope()->print();
system("Pause");
return 0;
}

Answer 1

Where to start? This has a lot of errors.
Don't have "using namespace std;", it clutters the global namespace. Rather, use std::list, std::cin, etc. using the namespace to identify the specific object or class.
In the exception class, don't write your own what() method. Just initialise the base class in the constructor.
class EPilaVacia : public std::exception
{
public:
EPilaVacia()
: std::exception("Error: Pila Vacía")
{
}
};
I assume that the class Pila is just a learning exercise. In real life you would use std::stack, not make your own.
If you are implementing the stack with a list, you don't need a "max" parameter.
Don't allocate the list dynamically, that is silly. Just use
std::list<T*> ila;
You don't need "cont". Use ila.size();
Don't make the functions like apilar() virtual. The list is private, so subclasses cannot access it, thus the methods cannot be overriden. Also, you don't have a virtual destructor, so inheritance is probably a bad idea.
void apilar(T t) is a disaster. You pass t by value, then store the address of the parameter, which then goes out of scope. The function is unnecessary, lose it.
Don't put "throw (EPilaVacia)" in method declarations. No compiler implements it and it is deprecated in the new C++11 standard.
In tope(), use ila.back(), not ila.pop_back().
T tope() const
{
if(ila.empty())
{
throw EPilaVacia();
}
else
{
return *ila.back();
}
}
In desapilar(), don't use clear as it will empty out the stack. Use something like this
T& desapilar()
{
if(ila.empty())
{
throw EPilaVacia();
}
else
{
T *pt = ila.back();
ila.pop_back();
return *pt;
}
}
NEVER use system("Pause"); Use std::cin.get();
The objects you allocate with new are never deleted. You have a memory leak.
There are probably more, but that should get you started.
Note: I've scrawled this down quickly. There are probably errors above, so check my code, don't just copy it.

Answer 2

Is this error happening on line "mystack=new stack<T>;" as that's the only line I see that might cause this. The reason for that is that mystack is defined as T*, not stack<T>. When the compiler tries to assign the new stack<T> to mystack, it sees that mystack is looking for T*, and says "I don't know how to make stack<T> into T*".
Now that you've fixed that error, you are getting a throw from nullptr exception. This would be best solved, normally, by running this under a debugger and seeing what line causes your program to behave poorly. However, by inspection, it appears that you're only pushing two things onto the stack, then try to use "top" to get a third: pfiguras2.tope()->print();.
You're also leaking memory, but Michael J's comment goes much better into the more nitpicky, less "make it not crash" area of this code.