Re: covariant return types and std::unique_ptr

=?ISO-8859-15?Q?Daniel_Kr=FCgler?= <>
Tue, 25 Sep 2012 22:03:37 -0700 (PDT)
Am 25.09.2012 22:33, schrieb Christof Meerwald:

I was just wondering the other day what the preferred way of rewriting
the following code fragment (using covariant return types) in C++11
(using std::unique_ptr for the return types) would be:


Obviously, just replacing the raw pointers with std::unique_ptr won't
work (without some additional casts) - but what's the cleanest way to
make it work (with the emphasis being on "cleanest")?

You might want to read the following thread where I responded to a
similar question:

In short: I would ensure that the actual virtual function still creates
a raw pointer (make this virtual function protected which is mostly
useful anyway and is known as "non-virtual interface" idiom), than make
provide non-virtual function (templates) that are implemented in terms
of the virtual functions, returning the smart pointer of your interest.

Example: Instead of your

   struct A
     virtual A* createAnother() = 0;


   struct ABase
     virtual A* doCreateAnother() = 0;

plus a template

   template<class Derived>
   struct A : ABase
     // Note: Not virtual!
     std::unique_ptr<Derived> createAnother() final {
       static_assert(std::is_base_of<A, Derived>(), "Need derived type");

Your derived types could now derive from the template instead:

   struct B : A<B>
     virtual B* doCreateAnother() override { ... }

See the referenced link for further possible extensions of that idea.

HTH & Greeting from Bremen,

Daniel Kr?gler

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