Good start Dominic! I have another example which I think using unique_ptr
is preferred.
We need to store a vector of pointers to the base class (which means we
have to use pointers here). However, storing raw pointer makes the
ownership tracking not explicit. What do you guys think?
// Base class.
class Transformation
{
public:
virtual ~Transformation() {}
virtual Try<Resources> apply(const Resources& resources) = 0;
};
// Derived class. Use composite pattern.
class CompositeTransformation : public Transformation
{
public:
CompositeTransformation() {}
* void add(std::unique_ptr<Transformation> transformation)*
* { *
* transformations.push_back(std::move(transformation));*
* } *
virtual Try<Resources> apply(const Resources& resources);
private:
*std::vector<std::unique_ptr<Transformation>> transformations;*
};
On Fri, Dec 12, 2014 at 3:01 PM, Dominic Hamon <[email protected]>
wrote:
>
> Great idea.
>
> There is one place that immediately springs to mind that I think is trivial
> but will get us used to the idea and exercise some of the demons: Unit
> tests. Within a test we regularly create some object on the heap and then
> delete it at the end of the test, or for the lifetime of a test fixture.
> All of these should be well scoped in lifetime and are prime targets for
> unique_ptr.
>
> On Wed, Dec 10, 2014 at 3:32 PM, Jie Yu <[email protected]> wrote:
> >
> > Dominic,
> >
> > Thank you for brining this up! Instead of making a decision based on high
> > level design and coding philosophy, I would really love to see some
> > concrete examples in our code base where we can start to use unique_ptr
> and
> > why it's better. Thoughts?
> >
> > - Jie
> >
> > On Wed, Dec 10, 2014 at 3:20 PM, Dominic Hamon <[email protected]>
> > wrote:
> >
> > > Hello!
> > >
> > > We have access to std::unique_ptr, as many of you know, but we're not
> > using
> > > it. I'd like to start using it. Before we do, I'd like to discuss how
> we
> > > use it, when we use it, and things to be aware of.
> > >
> > > The style guide has the following to say (long quote, sorry):
> > >
> > > "
> > > Prefer to have single, fixed owners for dynamically allocated objects.
> > > Prefer to transfer ownership with smart pointers.
> > >
> > > "Ownership" is a bookkeeping technique for managing dynamically
> allocated
> > > memory (and other resources). The owner of a dynamically allocated
> object
> > > is an object or function that is responsible for ensuring that it is
> > > deleted when no longer needed. Ownership can sometimes be shared, in
> > which
> > > case the last owner is typically responsible for deleting it. Even when
> > > ownership is not shared, it can be transferred from one piece of code
> to
> > > another.
> > >
> > > "Smart" pointers are classes that act like pointers, e.g. by
> overloading
> > > the * and -> operators. Some smart pointer types can be used to
> automate
> > > ownership bookkeeping, to ensure these responsibilities are met.
> > > std::unique_ptr is a smart pointer type introduced in C++11, which
> > > expresses exclusive ownership of a dynamically allocated object; the
> > object
> > > is deleted when the std::unique_ptr goes out of scope. It cannot be
> > copied,
> > > but can be moved to represent ownership transfer. std::shared_ptr is a
> > > smart pointer type that expresses shared ownership of a dynamically
> > > allocated object. std::shared_ptrs can be copied; ownership of the
> object
> > > is shared among all copies, and the object is deleted when the last
> > > std::shared_ptr is destroyed.
> > >
> > > It's virtually impossible to manage dynamically allocated memory
> without
> > > some sort of ownership logic.
> > > Transferring ownership of an object can be cheaper than copying it (if
> > > copying it is even possible).
> > > Transferring ownership can be simpler than 'borrowing' a pointer or
> > > reference, because it reduces the need to coordinate the lifetime of
> the
> > > object between the two users.
> > > Smart pointers can improve readability by making ownership logic
> > explicit,
> > > self-documenting, and unambiguous.
> > > Smart pointers can eliminate manual ownership bookkeeping, simplifying
> > the
> > > code and ruling out large classes of errors.
> > > For const objects, shared ownership can be a simple and efficient
> > > alternative to deep copying.
> > > Ownership must be represented and transferred via pointers (whether
> smart
> > > or plain). Pointer semantics are more complicated than value semantics,
> > > especially in APIs: you have to worry not just about ownership, but
> also
> > > aliasing, lifetime, and mutability, among other issues.
> > > The performance costs of value semantics are often overestimated, so
> the
> > > performance benefits of ownership transfer might not justify the
> > > readability and complexity costs.
> > > APIs that transfer ownership force their clients into a single memory
> > > management model.
> > > Code using smart pointers is less explicit about where the resource
> > > releases take place.
> > > std::unique_ptr expresses ownership transfer using C++11's move
> > semantics,
> > > which are relatively new and may confuse some programmers.
> > > Shared ownership can be a tempting alternative to careful ownership
> > design,
> > > obfuscating the design of a system.
> > > Shared ownership requires explicit bookkeeping at run-time, which can
> be
> > > costly.
> > > In some cases (e.g. cyclic references), objects with shared ownership
> may
> > > never be deleted.
> > > Smart pointers are not perfect substitutes for plain pointers.
> > > If dynamic allocation is necessary, prefer to keep ownership with the
> > code
> > > that allocated it. If other code needs access to the object, consider
> > > passing it a copy, or passing a pointer or reference without
> transferring
> > > ownership. Prefer to use std::unique_ptr to make ownership transfer
> > > explicit. For example:
> > >
> > > std::unique_ptr<Foo> FooFactory();
> > > void FooConsumer(std::unique_ptr<Foo> ptr);
> > > Do not design your code to use shared ownership without a very good
> > reason.
> > > One such reason is to avoid expensive copy operations, but you should
> > only
> > > do this if the performance benefits are significant, and the underlying
> > > object is immutable (i.e. std::shared_ptr<const Foo>). If you do use
> > shared
> > > ownership, prefer to use std::shared_ptr.
> > >
> > > Do not use scoped_ptr in new code unless you need to be compatible with
> > > older versions of C++. Never use std::auto_ptr. Instead, use
> > > std::unique_ptr.
> > > "
> > >
> > > Now I'm a big fan of explicit ownership and moving ownership rather
> than
> > > sharing non-smart pointers, but I recognise that in our code-base,
> > > ownership is difficult to reason about in many cases. However, we do
> have
> > > quite a few cases where we manage lifetime scope with explicit delete
> > calls
> > > and I'd like to start by eliminating those. Ie, using std::unique_ptr
> to
> > > manage lifetime not necessarily ownership.
> > >
> > > This is difficult though as we may pass these pointers to other methods
> > or
> > > even other libprocess processes. In cases that we can reason about the
> > > lifetime of the various pointers, that should be fine, but we have to
> be
> > > careful.
> > >
> > > One option is to start by replacing these pointers with process::Owned.
> > The
> > > downside to this approach is that it introduces more non-standard types
> > > and, because Owned is implemented using std::shared_ptr, doesn't move
> us
> > > closer to defining clear ownership.
> > >
> > > So the floor is open. Do we:
> > >
> > > 1) embrace std::unique_ptr, eliminate raw pointers except in rare,
> > > well-defined cases from a lifetime point of view?
> > > 2) eliminate raw pointers in favour of std::shared_ptr and
> > std::unique_ptr
> > > then work to eliminate the std::shared_ptrs
> > > 3) use process::Owned everywhere and then find places where we can
> > > transition them to std::unique_ptr
> > > 4) do nothing; there's no great benefit to std::unique_ptr and this
> smart
> > > pointer thing is just a fad.
> > >
> > >
> > > - dominic
> > >
> > >
> > > --
> > > Dominic Hamon | @mrdo | Twitter
> > > *There are no bad ideas; only good ideas that go horribly wrong.*
> > >
> >
>
>
> --
> Dominic Hamon | @mrdo | Twitter
> *There are no bad ideas; only good ideas that go horribly wrong.*
>
