Re: [boost] [smart ptr] Any interest in copy-on-write pointer for C++11?

Subject: Re: [boost] [smart ptr] Any interest in copy-on-write pointer for C++11?
From: Jeffrey Yasskin (jyasskin_at_[hidden])
Date: 2013-02-08 12:35:09

WebKit has a class vaguely like this for your case #2:
https://code.google.com/p/chromium/codesearch/#chrome/src/third_party/WebKit/Source/WebCore/rendering/style/DataRef.h
used at https://code.google.com/p/chromium/codesearch/#chrome/src/third_party/WebKit/Source/WebCore/rendering/style/RenderStyle.h&rcl=1360310731&l=137.
Semantically every copy is a real copy, and, contrary to Mathias'
assertion, couldn't be replaced by a move in C++11, but they want to
share identical values when doing so is cheap.

On Fri, Feb 8, 2013 at 7:16 AM, Ralph Tandetzky
<ralph.tandetzky_at_[hidden]> wrote:
> Hi,
>
> is there any interest in a copy-on-write pointer implementation? I wrote a
> cow_ptr<T> template class for C++11 which has the following use cases:
>
> 1. They are well suited as pimpl pointers for classes with value semantics.
> With most other smart pointers it is always necessary to reimplement the
> copy constructor, and the copy assignment operator, sometimes also the
> destructor.  This is not necessary here and you'll get the right semantics
> and even the additional performance gain due to lazy  copying. A futher
> advantage is that constness of cow pointer member functions propagates to
> the Impl object naturally making it easier to write const-correct code.
>             // central_class_with_value_semantics.h
>             class CentralClassWithValueSemantics
>             {
>             public:
>                 // ... public interface goes here ... //
>             private:
>                 struct Impl; // forward declaration
>                 cow_ptr<Impl> m;
>             };
>
>             // central_class_with_value_semantics.cpp
>             struct CentralClassWithValueSemantics::Impl
>             {
>                 // ... definition of hidden members goes here ... //
>             }
> This is called the pimpl idiom, or private implementation idiom, handle body
> idiom, or compiler firewall idiom.
>
> 2. For classes with members whose copy-operations are expensive and/or which
> take a lot of space in memory, these members can be wrapped in a cow
> pointer. An example are matrix or image classes whose data might be stored
> in a std::vector. The matrix header information may be changed without deep
> copy. This boosts performance and optimizes memory usage, but at the same
> time retains the value semantics one feels comfortable with.
>             class Matrix
>             {
>             public:
>                 // ... public interface goes here ... //
>             private:
>                 size_t nRows, nCols; // can be touched without deep copy.
>                 cow_ptr<std::vector<float>> data; // copy-on-write
>             }
> In such classes the default version of copy and move construction will
> usually work just fine.
>
> 3. You can add cloning to a class hierarchy from the outside. With
>             cow_ptr<Base> a( new Derived1 );
>             cow_ptr<Base> b( new Derived2 );
>             cow_ptr<Base> c;
>             c = a; // performs a shallow copy.
>             c->doSomething(); // makes a deep copy of a as a Derived1 class.
>                 // There is no slicing involved.
> you copy Base objects polymorphically. The class Base can even be abstract
> here. It is only required that Derived1 and Derived2 be CopyConstructible.
>
> 4. You can create arrays with elements that retain polymorphic behaviour and
> have genuine value sematics at the same time with
>             std::vector<cow_ptr<Base>>
> polymorphic_array_with_value_semantics;
>
> --
>
> There are probably other use cases. I published the code on github (
> https://github.com/ralphtandetzky/cow_ptr). Any suggestions for improvements
> would be greatly appreciated.
>
> Regards,
> Ralph Tandetzky.
>
>
> _______________________________________________
> Unsubscribe & other changes:
> http://lists.boost.org/mailman/listinfo.cgi/boost