CODEWITHSUNDEEP
c++
gsigms
gsigms

Reputation: 21

n2439 rationale behind "Extending move semantics to *this" limitations to non static member functions

What is the rationale behind n2439:

12.1.4 A constructor shall not be declared with a ref-qualifier.

and

12.4.2 A destructor shall not be declared with a ref-qualifier.

Assuming some class A with some costly ref counting semantics, i would write something like this, so as to be able to avoid retain/release-ing temporary objects:

struct B {
  A a;
  B (const& A a) & : a(retain(a)) {}
  B (const& A a) && : a(a) {}

  ~B () & { release(a); }
  ~B () && { }

  void do_something_with_a();
};

A a = getA();
auto b = B(a); // retain+released
b.do_something_with_a();       
B(getA()).do_something_with_a();  // no retain / release

Upvotes: 2

Views: 70

Answers (1)

Richard Hodges
Richard Hodges

Reputation: 69912

Intrusive reference counting like this is 20+ year old technology. It exists in such arcane places as COM and objective-c.

In their day these technologies were useful and helped us move forward (and a few steps back) but things have moved on. We have become wiser.

Nowadays we understand that separating the concerns of lifetime from function is a more sensible way to go. It allows decoupling of concepts, cleaner code that's easy to reason about.

This is why the standard has std::unique_ptr and std::shared_ptr...

...but no std::intrusive_base or std::intrusive_ptr.

Because these concepts are actually harmful.

Having reset our internal code-writing guidance systems so that we write nicely decoupled code, we find that we don't need to optimise constructors this way - if we want a reference to A we simply take a reference to A, if we want a copy of a shared_ptr to it, we simply copy the shared_ptr.

Code does what it says on the tin. There is no hidden magic. Suddenly, code with reference-counted objects can be reasoned about, we don't have to remember whether we need to release an object or not.

Welcome to the post c++11 world. It's wonderful :-)

Upvotes: 1

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