Re: Polymorphism without virtual in C++
On Aug 7, 1:59 am, Juha Nieminen <nos...@thanks.invalid> wrote:
feel wrote:
(Personally I really can't understand what's all the fuss
about the Pimpl idiom. It only makes classes consume more
memory and slower, which can be especially counterproductive
with small classes which have to be instantiated frequently
and in large amounts. The only situation where there may be an
advantage is if the class is large, it's copied around a lot,
and the Pimpl data is reference-counted or CoW'ed.)
It reduces coupling, sometimes enormously.
In my solution, the destructor of root class is not virtual, but we
can use base class pointer to point derived class object.
The base class destructor can destroy the data in the base class, yes.
I still fail to see the polymorphic part.
The real problem, as presented by others, is that if you ever add
anything in a derived class that needs to be destroyed, deleting the
object through a base-class pointer will most probably not destroy the
data in the derived class.
The real problem is that even if he never adds anything, it is
undefined behavior.
(Technically speaking deleting through a base class pointer without a
virtual destructor is undefined behavior even if the derived class is
empty, but I suppose most compilers will do what you expect.)
Most don't. (I'd expect an immediate program crash, but with
most, it will work most of the time, only crashing in specific
cases, or much later.)
[...]
class base
{
public:
~base()
{
delete[] p;
};
protected:
int *p;
base():p(new int[10])
{
};
base(int *pp) : p(pp)
{
};
};
You probably shortened the example for the sake of brevity, but a few
comments nevertheless:
- Allocating unmanaged memory in the initialization list of the
constructor is asking for trouble.
Why? In this case, he's doing it in the approved fashion:
passing the pointer immediately to the base class or member.
- If objects of this class are ever copied or assigned, problems will
happen.
- Deleting a pointer given to the constructor is dubious practice at
best. You can't know what it's pointing to.
That depends on the contract. Is boost::shared_ptr dubious
practice? It certainly deletes a pointer given to the
constructor.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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