C++ slicing caused by reference - c++

Question

I have problem to understand the behavior of the following code, which I modified from an example for C++ slicing:
#include <stdio.h>
#include <iostream>
struct B {
int x;
B() { x = 0; }
virtual void foo( const char* id ) {
std::cout << id << ": B=" << this << ", x=" << x << std::endl;
}
};
struct D1 : B {
int y;
D1() { x = 1; y = 100; }
virtual void foo( const char* id ) {
std::cout << id << ": D1=" << this << ", x=" << x << ", y=" << y << std::endl;
}
} d1;
struct D2 : B {
int z;
D2() { x = 2; z = 200; }
virtual void foo( const char* id ) {
std::cout << id << ": D2=" << this << ", x=" << x << ", z=" << z << std::endl;
}
} d2;
void main() {
std::cout << "d1 = " << &d1 << std::endl;
std::cout << "d2 = " << &d2 << std::endl;
std::cout << "By pointer at beginning: " << std::endl;
B* pb = &d1;
pb->foo( "pd1" );
pb = &d2;
pb->foo( "pd2" );
std::cout << "By Value: " << std::endl;
B b = d1;
b.foo( "d1" );
b = d2;
b.foo( "d2" );
std::cout << "By pointer after by value: " << std::endl;
pb = &d1;
pb->foo( "pd1" );
pb = &d2;
pb->foo( "pd2" );
std::cout << "By reference: " << std::endl;
B& rb = d1;
rb.foo( "rd1" );
rb = d2;
rb.foo( "rd2" );
std::cout << "By pointer after by reference: " << std::endl;
pb = &d1;
pb->foo( "pd1" );
pb = &d2;
pb->foo( "pd2" );
}
//The result is the following:
d1 = 0115B504
d2 = 0115B510
By pointer at beginning:
pd1: D1=0115B504, x=1, y=100
pd2: D2=0115B510, x=2, z=200
By Value:
d1: B=0036FE44, x=1
d2: B=0036FE44, x=2
By pointer after by value:
pd1: D1=0115B504, x=1, y=100
pd2: D2=0115B510, x=2, z=200
By reference:
rd1: D1=0115B504, x=1, y=100
rd2: D1=0115B504, x=2, y=100
By pointer after by reference:
pd1: D1=0115B504, x=2, y=100
pd2: D2=0115B510, x=2, z=200
From the above result, we can see that:
Value assignment causes slicing problem to the assign destination (b) by dropping the derived specific members, but leave the assign sources (d1 and d2) intact.
Reference assignment causes slicing to the assign destination (rd) by not assigning the derived specific members, thus partially changes the assign sources (d1 and d2).
At first, I was surprised that you can assign a reference to a different type (D2 to D1) via their base, until I realize the infamous C++ casting system. One conclusion seems that reference can only be initialized but not assigned.
We know that STL container for base object has slice problem because except list, all other STL container copy object around. It looks that STL container for base references should fair no better, unless it saves pointers inside.
How do you guys deal with this problem?
Thanks!
CP

Answer 1

I'm finding it hard to find a question in your text, but I'm going to assume it is something like:
"How do I polymorphically store objects in a standard container?"
First, let me just say that you can't store references in standard containers because you can't assign (rebind) them.
So the normal way is to have a container of base class pointers. If the container owns the items, then use smart pointers like unique_ptr or shared_ptr. If the container does not own the objects, then just use raw pointers.

Answer 2

B& rb = d1;
rb.foo( "rd1" );
rb = d2;
The last line is equivalent as if you wrote
d1 = d2;
except it applies only to base class subobject. In the first line you set up the reference creating rb as alias to d1 with different static type. The reference can not be reseated. The later assign targets the object. d1 directly could only be assigned with using trickery with its op=, but your changed static type makes Base::op= work.
Many mentors say you should only use abstract classes as base, and most of the reasoning points to accidents like this.
In hierarchies you rarely have the op= left, if absolutely needed you have some virtual ssign thing that verifies compatibility.