When I get a case of `slicing`?

Question

Let's look the next code (for example):

class A {
    int n;
public:
    int f() const { return n; }
    void set(int i) { n = i; }
};
class B : public A {
public:
    int g() const { return f()+1; }
};
void h(const A& a) {
    a.f();
}
int main() {
    B b;
    A& a = b;
    A* ptrA = new B; 
    h(b);
  delete ptrA;
    return 0;
}

Now, Let's look about these lines code:

A& a = b; // Is there "slicing"? (why?)
A* ptrA = new B; // Is there "slicing"? (why?)
A a = b; // Is there "slicing"? (why?)

I do not really understand when I want to use any of them, and when, alternatively, you will not be allowed to use one of them. What is really the difference between these lines..

You can look at [What is object slicing?](https://stackoverflow.com/questions/274626/what-is-object-slicing) for examples. Note that none of the answers there actually explain what is the problem with it (including fgp's answer, even though it claims otherwise). — user7860670, Mar 15 '18 at 10:05
kudos for your patience. Actually I dont know what the SO etiquette says about deleting a closed question to repost it, but I am glad to see that you managed to improve your question considerably and that eventually you got an answer. — 463035818_is_not_an_ai, Mar 15 '18 at 10:24

Sean · Accepted Answer · 2018-03-15T10:08:55.803

6

Slicing is when you assign a derived object to a base instance. For example:

B b;
A a = b;

The issue here is that the copy constructor for A that is being called only sees the A part of B, and so will only copy that. This is an issue if your derived class has added additional data or overridden the behaviour of A.

When you assign an object to a reference or a pointer there is no slicing. So

A &a = b;

Is fine, as is:

A *a = &b;

edited Mar 15 '18 at 10:08

answered Mar 15 '18 at 10:06

Sean

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6

The opposite case is slicing as well: `A a; B b; A & ab = b; ab = a;` – user7860670 Mar 15 '18 at 10:08
And what is really the meaning of `A &a = b` in this case? all the fields of B class (including these that it's inherit from `A`) will be belongs to `a`? – StackUser Mar 15 '18 at 10:10
@VTT Can you explain your example please? – StackUser Mar 15 '18 at 10:11
@StackUser `A & ab = b;` is just a reference binding, slicing happens at the `ab = a;` because `ab` is a reference to an instance of `B` class. – user7860670 Mar 15 '18 at 10:13
@VTT So the type of `ab` is `B` ? – StackUser Mar 15 '18 at 10:14
@StackUser The type of `ab` is `A &` but it references the `A` object that is inside of `B` object instance. – user7860670 Mar 15 '18 at 10:15
@VTT I only want to understand the concept. What is the difference between `A a1;` to `A& a2 = b;` (while `B b;`) (Apart of the types of them) ? – StackUser Mar 15 '18 at 10:33
@StackUser `a1` is an object of `A` class, `a2` is just a reference to an object of `A` class. – user7860670 Mar 15 '18 at 10:36
@VTT Ok, and what for example I can doing with `a1` that I can't with `a2` ? – StackUser Mar 15 '18 at 10:37
@VTT In addition, in your example: `ab=a` what is deleted from `a`? – StackUser Mar 15 '18 at 10:46
@StackUser In both cases `a = b;` and `ab = a;` nothing gets deleted. Just not all the fields are copied. – user7860670 Mar 15 '18 at 11:22
which fields are not copied in this case `ab=a` ? – StackUser Mar 15 '18 at 11:30

When I get a case of `slicing`?

1 Answers1