Re: Raw pointers not evil after all?
On 5/1/2013 3:29 PM, Christopher Pisz wrote:
Snip
Consider this illustration snippet:
class Owner {
private:
std::vector<Owned *> owns;
public:
...
~Owner();
void addSomething(Owned *);
};
...
~Owner()
{
for(std::vector<Owner *>::iterator it(owns.begin()); it !=
owns.end(); ++it)
delete (*it);
}
void Owner::addSomething(Owned *smth)
{
owns.push_back(smth);
}
void f()
{
Owner oneOwner, twoOwners;
Owned *smthToOwn = new Owned;
oneOwner.addSomething(smthToOwn); // BUT WAIT! The pointer to
smthToOwn is still in f()!
twoOwners.addSomething(smthToOwn); // FATAL!
}
What I notice is this:
1. Only one Owner can own a given Owned. Yet with a raw pointer,
there's nothing to stop us from putting it into two Owners, causing a
fatal crash when both are destroyed. Now, one could say "well then
just don't do that", but shouldn't that be enforced by code and not
just the user's understanding?
2. This concern suggests to use auto_ptr or something like it. But
auto_ptr is attacked on the same website. And technically, the
auto_ptr owns the object, not the Owner. It seems the only way Owner
can directly own the object is with an _EVIL_ raw pointer.
So what to do?
P.S
In your example above void f() allocated the object and therefore should
own it. Design problem. Write Owner in such a manner that it allocates
the object and owns it.
Owner::Add(params){...//allocates and stores}
const Owned & Owner::Get(params){...//returns a const reference or
reference to the stored object}
Better yet, since you are just wrapping a std::vector, implement
Owner::iterator and Owner::const_iterator. Look at what the vector
itself does. Does it return a pointer? Nope. Does it own its elements?
Yep. Can users get and modify the contained object? Yep. How does it do
that? :)
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