What happens if I "slice" to an abstract class - c++

Question

First off, I know the assignment operator cannot be defined in a class that has some subclasses. I understand it is because we don't want to make Subclass1 = Subclass2 possible.
But let's assume Class is an abstract class and Subclass is its... ya know. Then, is it feasible to do something like this?
Class* p = new Subclass;
Subclass s1;
*p = s1;
Actually, I tried implementing that in my code, but it didn't work :)
Could you please help?
My full code:
#include <cstdlib>
#include <iostream>
#include <typeinfo>
using namespace std;
class BadIndex{
int index;
public:
BadIndex(int i):index(i){}
int getIndex(){ return index; }
};
template <typename t>
class Wielomian{
public:
~Wielomian(){}
virtual int getDeg() = 0;
virtual t& operator [](int) = 0;
virtual bool operator ==(Wielomian<t>&) = 0;
virtual Wielomian<t>& operator +(Wielomian<t>&) = 0;
virtual Wielomian<t>& operator +=(Wielomian<t>&) = 0;
};
template <typename t>
class TabWiel: public Wielomian<t>{
int deg;
t* plnml;
public:
TabWiel(t tab[] = {}, int size = 0);
~TabWiel();
TabWiel(const TabWiel<t>&);
TabWiel<t>& operator =(const TabWiel<t>&);
template <typename st>
friend ostream& operator <<(ostream& s, TabWiel<st>& tw);
int getDeg(){ return deg; }
t& operator [](int);
bool operator ==(Wielomian<t>&);
TabWiel<t>& operator +(Wielomian<t>&);
TabWiel<t>& operator +=(Wielomian<t>&);
};
template <typename t>
TabWiel<t>& TabWiel<t>::operator =(const TabWiel<t>& tw){
if (this != &tw){
delete[] plnml;
deg = tw.deg;
plnml = new t[deg + 1];
for (int i = 0; i < deg + 1; i++)
plnml[i] = tw.plnml[i];
}
return *this;
}
template <typename t>
TabWiel<t>::TabWiel(t tab[], int size){
deg = size - 1;
plnml = new t[deg + 1];
for (int i = 0; i < deg + 1; i++)
plnml[i] = tab[i];
if (deg == -1){
deg = 0;
plnml[0] = 0;
}
}
template <typename t>
TabWiel<t>::~TabWiel(){
delete[] plnml;
}
template <typename t>
TabWiel<t>::TabWiel(const TabWiel<t>& tw){
deg = tw.deg;
plnml = new t[deg + 1];
for (int i = 0; i < deg + 1; i++)
plnml[i] = tw.plnml[i];
}
template <typename t>
t& TabWiel<t>::operator [](int s){
if (s >= 0 && s < deg + 1)
return plnml[s];
else
throw BadIndex(s);
}
template <typename t>
bool TabWiel<t>::operator ==(Wielomian<t>& tw){
try{
TabWiel<t>& rhs = dynamic_cast<TabWiel<t>&>(tw);
if (deg == rhs.deg){
for (int i = 0; i < deg + 1; i++){
if (plnml[i] != rhs.plnml[i])
return false;
}
return true;
}
return false;
}
catch (const bad_cast& e){
cerr << "An exception" << e.what() << " thrown." << endl;
}
}
template <typename t>
ostream& operator <<(ostream& s, TabWiel<t>& tw){
for (int i = 0; i < tw.deg + 1; i++){
if (i != tw.deg)
s << tw.plnml[i] << "x^" << i << "+";
else
s << tw.plnml[i] << "x^" << i << endl;
}
return s;
}
template <typename t>
TabWiel<t>& TabWiel<t>::operator +(Wielomian<t>& tw){
try{
TabWiel<t>& rhs = dynamic_cast<TabWiel<t>&>(tw);
if (rhs.deg <= deg){
for (int i = 0; i < rhs.deg + 1; i++)
plnml[i] = plnml[i] + rhs.plnml[i];
return *this;
}
else{
t* tmp = new t[deg + 1];
for (int i = 0; i < deg + 1; i++)
tmp[i] = plnml[i];
int tmp_deg = deg;
delete[] plnml;
deg = rhs.deg;
plnml = new t[deg + 1];
for (int i = 0; i < deg + 1; i++){
if(i < tmp_deg + 1)
plnml[i] = tmp[i] + rhs.plnml[i];
else
plnml[i] = rhs.plnml[i];
}
return *this;
}
}
catch (const bad_cast& e){
cerr << "An exception" << e.what() << " thrown." << endl;
}
}
template <typename t>
TabWiel<t>& TabWiel<t>::operator +=(Wielomian<t>& tw){
try{
TabWiel<t>& rhs = dynamic_cast<TabWiel<t>&>(tw);
TabWiel<t>* nowy = new TabWiel<t>;
TabWiel<t> copy;
copy = *this;
*nowy = copy + rhs;
return *nowy;
}
catch (const bad_cast& e){
cerr << "An exception" << e.what() << " thrown." << endl;
}
}
I wish the assignment of *p to non-empty subclass object worked. But it doesn't - all the code does, is that it enters "Wielomian" definition and then proceeds to the next line of the main function (which in my case is the last line).

Answer 1

Your question is very interesting.
First of all, your code doesn't work because of slicing: you have two objects of Subclass, but
the compiler thinks that one of it is only a Class. So the code generated copies only the common
part of the data.
To demonstrate this, let's ellaborate on gha.st 's initial code extract:
struct Class { int a; virtual void hugo() = 0; };
struct Subclass : Class { int b; void hugo() override { cout<<"sub"<<a<<b<<endl; } };
int main() {
Class* p = new Subclass;
static_cast<Subclass*>(p)->a = 2;
static_cast<Subclass*>(p)->b = 3;
Subclass s1;
s1.a = 4; s1.b=5;
*p = s1; // slicing !!
p->hugo();
return 0;
}
What happens here ? Well, b member isn't copied, although *p is in reality a Subclass !
But *p is still a Subclass, so we could use polymorphism to get this work. The trick is to use an virtual clone() member to clone
an object (the object shall know its own type) into a target, if the target has the same type.
Then you could define operator=() for Class to use this clone(). This makes it handy to use, but the drawback is that you'll no longer
be able to rely on default operator= for any descendent of Class if you want to avoid an endless recursion.
Here the proof of concept:
struct Class {
int a;
virtual void hugo() = 0;
virtual bool clone(Class*t) = 0;
Class& operator=(Class& o) {
if (!o.clone(this)) { // try to clone on subclass on a target of same subclass
// here,the source and target might differ. Only common members can be copied
a = o.a;
}
return *this;
}
};
struct Subclass : Class {
int a,b;
void hugo() override { cout<<"sub"<<a<<b<<endl; }
bool clone(Class*t) {
cout<<"Clone ";
if (dynamic_cast<Subclass*>(t)) { // if source and target of same subclass
//*dynamic_cast<Subclass*>(t) = *this; // this doesn't work cause default operator will try to copy the Class base, causing recursion
dynamic_cast<Subclass*>(t)->a = a; // copy members
dynamic_cast<Subclass*>(t)->b = b;
return true;
}
else return false; // or tell that class of source and target are different.
}
};
Then you can use the main() function above, and see that the object is properly copied.
This trick is a kind of simplified double dispatch. You could even elaborate further by foreseing various kind of conversions depending on source and target subtype.