Re: Template setters

=?ISO-8859-1?Q?=D6=F6_Tiib?= <>
Tue, 10 Sep 2013 22:14:49 -0700 (PDT)
On Tuesday, 10 September 2013 10:50:02 UTC+3, Kamil Rojewski wrote:

Lets see your setter: First type falls aside (it has both 'operator='
deleted, but it may be is cloned in setter). It also feels
needless complication to have such template for next two kinds since
one can use only 'T const&' and other only 'T&&'. So only the last
kind (that has both 'operator='s intact) can benefit from the template.

You are right on this one. When a type is only copyable or only
movable, this approach seems unnecessary. But then we are relying on
the knowledge of that type, while making a function contained in
another class. What if the type suddenly became copyable/movable?

That is the logical problem that I try to bring your attention at.
Most types that lack some ability should lack it. The reason is
conceptual nature of type. Some things (like thread, file descriptor)
are not copyable. Others (like some atomic) are not movable. Numerous
things (most dynamically polymorphic objects) are neither. That is
because of the very nature of the objects and not because of strange
whim of designers.

Using a template would ensure future-correctness in a type-agnostic
way. It could be viewed as a form of extensibility (not much, but
still) and reducing coupling. In this case, the question boils down to
assessing if such approach has merit for a given type. If we take into
account the last example with both copy and move, we could end up with
templated and non-templated versions, based on this assessment. I
think it warrants further analysis.

It is needless complication added to code if it anticipates that
something that is highly unlikely (a thread becomes copyable) may
happen. C++ is strongly typed. Interfaces dictate types, generic
interfaces dictate conceptual nature of types (with traits and
'enable_if's). "Anything goes" is not good policy for C++.

Lets see old way: We can easily have 'T const&' for each kind. That
makes 'T const&' lot better candidate for uniformity.

Yes and no. It doesn't work for non-copyable, but movable types
(unique_ptr for example).

Maybe 'unique_ptr' is bad example since it is usually conceptually
not attribute itself but merely a "unique reference" to actual
attribute. I trust it must work with 'T const&'. Anything must.
Does not below code work? (I haven't tested myself but it feels
it should):

   #include <memory>
   struct X {};

   typedef std::unique_ptr<X> XPtr;

   XPtr theX;

   void setTheX( XPtr const& x )
       theX.reset( x ? new X(*x) : nullptr );

   int main()
       XPtr someX( new X );
       setTheX( someX );
       XPtr noX;
       setTheX( noX );

For everything else, it's sufficient. But
it doesn't benefit from move semantics and logical implicit
conversions by the use of operator = overloading, like
operator =(nullptr_t) in unique_ptr.

I do not suggest to use 'T const&' as uniform way to pass parameters.
I merely pointed out that it is better candidate for such uniformity.
For me it is fine when conceptually different parameters are passed
differently and that the interfaces make it clear.

Side question: Why exactly only the "setters" for that kind? /Any/
function in what copying or assignment of parameter is made /may/
benefit from that trick.

Setters are explicitly designed to set and only set a field in a class
to the value supplied by the parameter. Using this trick on other
methods, exposes their inner workings and breaks encapsulation.

How? You turn something in interface into generic (template) and that
somehow exposes something and breaks encapsulation? For me it loosens
interface (might be overly so designer likely has to add trait checks)
and does not expose nor break anything.

Even more - it makes them constrained (quite ironic), since they
would /have to/ always allow different types as input, while the
implementation might change in a way that only one concrete type is
supported, thus not being able to enforce type-correctness.

Oh. Template in interface does not mean that "anything goes"?
I just asked why /only/ setters. I myself use that trick /only/ when:
 0) the type is movable and copyable /and/
 1) it is clear that move will help (for example profiling shows) /and/
 2) the bodies of move and copy overloads would be exactly same /and/
 3) I do not want to burden callers with always making movables for
    move-only version of interface.
That reads "rarely".

After all, we code against interfaces, not implementations.

That is true and that is why I dislike "setters". Too exposing and
intrusive (forces users to deal with other classes internal details)
interface for my taste.

Had the problem been in template I would just suggest two overloads
like 'operator=' has. The problem is that it will logically complicate
things because the nature of different IN parameters is different.
Removing that difference means making most types moveable and copyable
and that feels wrong (about as wrong as making most member functions

I disagree on the part about "making" a type moveable and copyable.
The type is left alone; there is no need of any knowledge about it at
the setter level. This approach is completely safe in this regard as
no knowledge about the type is assumed.

Here I am confused. Lets see:

  template<class T>
  void setWidget(T &&widget) // <- until here too few knowledge.
      mWidget = std::forward<T>(widget); // <- errors here about no
                                         // suitable 'operator='
                                         // for internal design
                                         // detail. Too much
                                         // knowledge required.

The user of interface needs not only know types that can be passed in
but what the heck is that 'mWidget' and what set of 'operator=' its
type has. How it is "there is no need of any knowledge about it"?

Making two overloads requires the type to be able to accept the two
different input parameters we incorporate in the setters.
That type would be constrained by our
setters and our class would also be constrained by the type. Removing
the ability to be copied or moved would break every line that uses
such functionality explicitly, but would also break every line that
uses our setters. And if the type would suddenly get copyable or
movable, we would not benefit from such change, unless we monitor that
type (introducing coupling/constraints), make new setters (changing
interface) and possibly change the uses of the previous setters to
take advantage of the new features (pretty much impossible and
generally bad in principle as it's the role of the class' users).

Such abilities like "can be moved" or "can be copied" are part of
conceptual nature of type. If something like that changes in interface
then that is major change. Such things should not change out of blue.
Also like I pointed out the possible problem diagnostics are just moved
from functions signature into body of the setter so that can be hardly
viewed as a good solution for changes like you describe.

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