mask copy constructor

[zio_mangrovia]

About this code, I'm not understanding because it has been defined this copy constructor
"Anfiteatro(const Anfiteatro &)", I think to avoid to have similar events:
Anfiteatro aa=a2;
Compilator detects copy constructor with this header and it make nothing because function is not defined, but why!?!? What is need?! But I'n not understanding.



this is code:

compito.h

#include <iostream>
using namespace std;

class Anfiteatro{
   
   struct elemento{
      int numMatt;
      elemento * succ;
   };

   elemento* testa;
      
   // mask copy constructor
   Anfiteatro(const Anfiteatro &);

   // funzioni di utilita'   
   void elimina();

public:

   // PRIMA PARTE
   Anfiteatro(int);
   bool aggiungiMattonelle(int);
   void aggiungiColonna(int);
   friend ostream& operator<<(ostream&, const Anfiteatro&);

   // SECONDA PARTE  
   bool togliColonna(int);
   Anfiteatro& operator= (const Anfiteatro&);
   ~Anfiteatro(){elimina();}
};

compito.cpp

#include "compito.h"

// funzione di utilita'
void Anfiteatro::elimina(){    
    elemento* p=testa;
    while(testa!=0){   
	   testa=p->succ;
       delete p;
       p=testa;
    }
}


// --- PRIMA PARTE ---

Anfiteatro::Anfiteatro(int n){
    if (n < 0) 
         n=0;
    testa = 0;
    elemento* q;
    for (int i=0; i<n; i++){  
        q = new elemento;
        q->numMatt = 0;
	 q->succ = testa;
        testa = q;
    }
}

bool Anfiteatro::aggiungiMattonelle(int k) {
    if (k < 0)
        k = 0;
    elemento* pmin = testa;
    if (testa == 0)
        return false;
    for (elemento* p = testa; p != 0; p = p->succ)
        if ( p->numMatt < pmin->numMatt )
            pmin = p;
    pmin->numMatt += k;
    return true;
}

void Anfiteatro::aggiungiColonna(int k) {
    if (k <0)
        k = 0;
    elemento *p, *q;
    for (p = testa; p != 0; p = p->succ)
        q = p;

    p = new elemento;
    p->succ = 0;
    p->numMatt = k;
    if (testa == 0)
        testa = p;
    else
        q->succ = p;
}


ostream& operator<<(ostream& os, const Anfiteatro& a){   
    os<<'<';
    for (Anfiteatro::elemento* p=a.testa; p!=0;p=p->succ){   
        os<<'['<<p->numMatt<<']';
        if (p->succ!=0)
            os<<',';
    }
    os<<'>';     
    return os;
}



// --- SECONDA PARTE ---


bool Anfiteatro::togliColonna(int n) {
    elemento* p, *q;
    int cont = 1;
    if (n <= 0)
	return false;

    for (p = testa; p != 0 && cont<n; p = p->succ) {
	  q = p;
	  cont++;
    }
    if (p == 0)
        return false;
    if (p == testa)
        testa = p->succ;
    else
        q->succ = p->succ;
    delete p;
    return true;
}

Anfiteatro& Anfiteatro::operator=(const Anfiteatro& a){    
    if ( this != &a){  
	 elimina();
        testa=0;
        elemento *q;
        for (elemento* p=a.testa; p!=0;p=p->succ){   
	     elemento* r=new elemento;
            r->numMatt = p->numMatt;
            r->succ=0;
            if (testa==0)
                testa=r;
            else
                q->succ=r;
            q=r;
        }
    }
    return *this;
}

main.cpp

#include "compito.h"
#include <iostream>
using namespace std;

int main()
{       
   cout << endl << "--- PRIMA PARTE ---" << endl;

   cout<<"\nChiamata al costruttore"<<endl;
   Anfiteatro a(4);
   cout<<a<<endl;

   cout << "Chiamata alla funzione aggiungiMattonelle()" << endl;
   a.aggiungiMattonelle(7);
   cout << a << endl;

   cout << "Altra chiamata alla funzione aggiungiMattonelle()" << endl;
   a.aggiungiMattonelle(12);
   cout << a << endl;
   
   cout << "\nChiamata alla funzione aggiungiColonna()" << endl;
   a.aggiungiColonna(8);
   cout << a << endl;   


   cout << endl << "--- SECONDA PARTE ---" << endl;

   cout << endl << "Chiamata alla funzione togliColonna()" << endl;
   a.togliColonna(1);
   cout << a << endl;

   {
       cout << endl << "Test dell'operatore assegnamento " << endl;
       Anfiteatro a1(2);
       cout << "a1 ( *prima* dell'assegnamento ) " << a1 << endl;		
       a1 = a;
       cout << "a1 ( *dopo* l'assegnamento ) " << a1 << endl;		
       cout << endl;
       cout << "Test del distruttore (a1 sta per essere distrutto)" << endl;		
   }

   return 0;

}

thanks

[TubularX]

This is OK. Declaring the copy constructor private without defining it, is an old trick to prevent copies. (in modern C++, "= delete" can be used instead).

The Anfiteatro class has a pointer (testa) that needs to be deleted in the destructor. If shallow copies were allowed, it would be dangerous because the destructor may then delete the same memory more than once.

[zio_mangrovia]

If shallow copies were allowed, it would be dangerous because the destructor may then delete the same memory more than once.

To make an example:

If I defined "Anfiteatro dest=source",
default copy constructor should to copy testa pointer content of source class object to testa pointer of dest class object,
so both class objects will share the same area memory and the destructor may then delete the same memory more than once.
Is't right?

[zio_mangrovia]

I have another question about this code please.
The "constructor" Anfiteatro::Anfiteatro(int), I could declare it as Anfiteatro::Anfiteatro(const int) because value which is passed to constructor is always numeric costant.
Thanks

[VictorN]

I have another question about this code please.
The "constructor" Anfiteatro::Anfiteatro(int), I could declare it as Anfiteatro::Anfiteatro(const int) because value which is passed to constructor is always numeric costant.
Thanks

It doesn't matter and is not needed.
just because the int argument is always passed by value (creating a copy)!

[TubularX]

Right, the copied object will have a pointer that points to the same memory.

If you really want to make a copy, you would either need to make a deep copy (i.e. make sure the memory pointed to is copied as well), or use a reference counting smart pointer like std::shared_ptr. Whether that is a good idea or not in this case is another question.

You can make the int parameter const if you want to express intent that the value must not be modified. But that's not currently the case, because of the line "n=0;" inside the constructor.

[zio_mangrovia]

I don't understand because main.cpp says:

#include "compito.h"
#include <iostream>
using namespace std;

Compito.h contains just both #include <iostream> and using namespace std;
I thought it was necessary #include "compito.h"

[2kaud]

I don't understand because main.cpp says:

#include "compito.h"
#include <iostream>
using namespace std;

Compito.h contains just both #include <iostream> and using namespace std;
I thought it was necessary #include "compito.h"

If that is what Compito.h contains, then either you don't need to include this in .cpp - or you don't need the #include and using statements in .cpp. However note that for good practice, using namespace shouldn't be part of a .h.