C++ "Bus error: 10" and working with pointers - c++

Question

I'm doing a Data Structures exercise and I have been blocked since yesterday with a bus error, which I reckon is because I'm doing bad things with the memory. But I cannot figure out what exactly.
These are the requirements that I have established for the practice:
able to add a product (any way will do) to the list
able to retrieve the product in the list at the current position (next, prev, moveToStart, moveToEnd… there's cursor pointer, called "actual" here)
any changes I do to the retrieved product should be updated in the data structure (ie. list::retrieve(*product), product->visits++)
This is the code that I have. Apologies about the var names, I have to do it in spanish and therefore names are in spanish.
class producto { // My product
public:
string marca;
double precio;
int visitas;
int compras;
producto () {}
producto (string M, double P, int V = 0, int C = 0) : marca(M), precio(P), visitas(V), compras(C) {}
};
class nodo {
public:
producto valor; // value
nodo *siguiente; // next
nodo *anterior; // prev
nodo (producto P, nodo *A = NULL, nodo *S = NULL) : valor(P), anterior(A), siguiente(S) {}
};
class lista {
private:
nodo *inicio;
nodo *final;
nodo *actual;
public:
lista();
bool esta_vacia(); // is empty?
bool es_final(); // is the end?
int insertar(producto p); // insert given p
void moverPrincipio(); // "move to beginning"
void siguiente(); // "next"
void imprimir(); // "print"
int leer(producto *p); // read, return 0 or 1 if successful, return product by ref
};
lista::lista() {
this->inicio = NULL;
this->final = NULL;
this->actual = NULL;
}
bool lista::esta_vacia() {
return (this->inicio == NULL);
}
bool lista::es_final() {
return (this->actual == NULL);
}
void lista::moverPrincipio() {
this->actual = this->inicio;
}
void lista::siguiente() {
if(!this->es_final()) {
this->actual = this->actual->siguiente;
}
}
void lista::imprimir() {
int i = 1;
producto *p;
this->moverPrincipio();
while(!this->es_final()) {
if(this->leer(p) == 0) {
cout << i << ".- ##" << p->marca << "##, Views ##" << p->visitas << "##\n";
p->visitas++;
i++;
this->siguiente();
}
}
}
int lista::leer(producto *p) {
if(this->actual != NULL) {
*p = this->actual->valor;
return 0;
} else {
return 1;
}
}
int lista::insertar(producto p) {
if(this->esta_vacia()) {
nodo *tmp = new nodo(p);
this->inicio = tmp;
this->final = this->inicio;
} else {
nodo *tmp = new nodo(p, this->final);
this->final->siguiente = tmp;
this->final = tmp;
}
return 0;
}
I have removed unnecessary code. This is how I'm using it (and failing miserably):
lista *productos = new lista();
productos->insertar(producto("Shoes", 19.90));
productos->insertar(producto("Socks", 25.00));
// I should expect views = 0
productos->imprimir();
// But now, views = 1
productos->imprimir();
Upon execution, the only thing I get is "Bus error: 10" when doing imprimir ("print"), the first time. Insertion works without errors (but something could be wrong there too).
My idea is to hold the product inside the node, and give a reference to its location when returning it, so that any changes are reflected there too (for example, increase the view or purchase counter of a retrieved element, reflects the change when reading the list later).
I'd be extremely thankful if someone could point out the mistakes I'm doing here.
Thanks!!
UPDATE Here's a compilable example.

Answer 1

You pass a pointer to lista::leer and you want to write a value to it. You will be writing in unallocated memory. Probably, what you wanted was a pointer to the actual element.
First of all, you need to modify the signature:
int lista::leer(producto **p);
note the double star, since we will be writing the pointer itself.
Then, you have to assign a pointer to actual->valor to it in lista::leer:
*p = &(this->actual->valor);
Finally, you have to pass a pointer to p in lista::imprimir:
if(this->leer(&p) == 0) {
// ...
}
Alternatively, you might modify lista::leer to return a pointer and check if it is nullptr/NULL.