Re: rvalue reference factory?

SG <>
Thu, 22 May 2014 07:25:46 -0700 (PDT)
On Tuesday, May 13, 2014 12:50:00 AM UTC+2, wrote:

    Derived1 && build(void)
        return std::move(Derived1());

I'd like to stress that rvalue references are not some magical powder
that you can spinkle on something to make it OK to return references
to function-local objects. The compiler does not do anything magical.
It returns a reference and this reference immediately becomes a
dangling reference because the temporary object ceases to exist.

Also, I'd like to mention that the use of std::move in cases where it
is not necessary, is a bad idea. In this case, it pretty much disables
the compiler warning about returning a reference to a function-local
object because the compiler is not aware of std::move returning
something that refers to the same object as its parameter does. (If
you're interested in a language that tracks lifetimes at compiletime
as part of the type system, you might want to look into Rust.)
Specifically, in a return statement with a temporary or function-local
object std::move is not necessary and actually prevents the RVO

If you're interested in writing a class that should be able to move,
it's your job as a class author to tell the compiler via a move
constructor how a move is supposed to happen -- or better yet: Try
to write the class in a way that the special functions are created for
you by the compiler (referred to as "rule of zero" these days).

However, in your case you're interested in runtime polymorphism. This
demands an indirection, yes, but not using a reference as return type
for your factory functions. The natural choice would be a unique_ptr
from the std namespace via #include <memory>:

   unique_ptr<Base> build(string const& what);

And if you don't have a special reason to turn your factory into a
class, then you should probably use a function. To quote John Carmack,
"Sometimes the elegant implementation is just a function. Not a
method. Not a class. Not a framework. Just a function."


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