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.*
