rvalue references

[Marco Leise via Digitalmars-d]

Am Tue, 18 Oct 2016 22:43:01 +0200
schrieb Timon Gehr <timon.gehr at gmx.ch>:

> It wouldn't even be the same thing if it was allowed. D const is not C++ 
> const. Enforcing transitive read-only on rvalue references does not make 
> that much sense.

For me using const ref vs. const is typically entirely a
matter of avoiding a costly copy. The function body shouldn't
care whether the data came from an rvalue or lvalue. It is
exemplified in the following lines where doSomething takes a
const ref and worldTransform returns by value:

  const worldTransform = someObj.worldTransform();
  otherObj.doSomething(arg1, arg2, worldTransform);

I'd prefer to write:

  otherObj.doSomething(arg1, arg2, someObj.worldTransform());

as the temporaries add significant noise in some functions and
make the equivalent C++ code look cleaner.

It may be noteworthy that the lack of rvalue references only
really got annoying for me with matrix calculations, because
there are so many medium sized temporaries. Some of them come
directly from binary operators. Functions often need to
perform a set of computations that bare a lot of similarity,
where visual cues help the understanding. In a reduced form:

  calculate(  matrix);  // works
  calculate(2*matrix);  // doesn't work, requires temporary
  calculate(3*matrix);  //              "

Introducing temporaries makes the similarity of the
calculation less evident. Making calculate take auto-ref would
result in duplicate code and thrash the instruction cache
(especially with 2 or 3 arguments). Dropping "ref" results in
unnecessary matrix copies at this and other call sites.

-- 
Marco