Re: overriding default copy-by-value semantics across a hierarchy of classes
Paul Bibbings wrote:
<snip - example of a hierarchy of two classes, each managing a raw
pointer to allocated storage >
Now all that is needed is to define the copy constructor and
assignment operator for MyDerived, and I'm wondering whether
the following would be the best way to do this:
MyDerived::MyDerived(const MyDerived& md)
: MyBase(md), b_ptr(new B(*md.b_ptr))
{ }
MyDerived& MyDerived::operator=(const MyDerived& md) {
if (this != &md) {
MyBase::operator=(md);
delete b_ptr;
b_ptr = new B(*md.b_ptr);
}
return *this;
}
Adding debug print statements suggests that these work as I would
want them to,
If you ignore what happens if an exception gets thrown in the assignment
operator, these definitions indeed work.
but somehow the assignment operator in particular
looks a little clunky in needing to use the explicit function call
syntax for the base operator=. Are there perhaps other ways that
I have missed to define these last two functions that might be
better, even assuming that the general strategy is correct in the
first place?
The recommended idiom for writing assignment operators is called the
copy-and-swap idiom.
For this idiom, your classes need a swap member to cheaply swap the
contents of two objects:
void MyBase::swap(MyBase& other)
{
using std::swap;
/* swap the members */
swap(this->a, other.a);
}
void MyDerived::swap(MyDerived& other)
{
using std::swap;
/* swap the members */
swap(this->b, other.b);
/* swap the base-class portions */
MyBase::swap(other);
}
Then you can write the operator= like this:
MyBase& MyBase::operator=(const MyBase& mb)
{
/* create a copy of the object to assign */
MyBase copy(mb);
/* swap the copy with *this */
mb.swap(*this);
return *this;
}
MyDerived& MyDerived::operator=(const MyDerived& md)
{
/* create a copy of the object to assign */
MyDerived copy(md);
/* swap the copy with *this */
md.swap(*this);
return *this;
}
As an alternative implementation of this idiom, you can use pass-by-
value instead of pass-by-reference and explicitly making a copy.
Regards
Paul Bibbings
Bart v Ingen Schenau
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