Often, one wants to call a function with a one-off callback. There are
currently three ways to do this:


1. templates


   template <typename Func>  // void (int, int)

   void foo(int a, int b, Func func);


The problem here is that foo's definition must be available, and there
is an increase in compile time, executable size, and the odds of seeing
a cryptic error (to be reduced with concepts).


2. std::function


   void foo(int a, int b, std::function<void (int, int)> func);


We've solved the problems of the template above, but added a virtual
function call, an allocation (which can be avoided be the
implementation, sometimes), and a requirement that the lambda we send
here be copyable; if we pass func down the line, we add either extra
indirection or extra copies/allocations.


3. custom abstract class


   class foo_callback {

   public:

     virtual ~foo_callback() {} // not really necessary

     virtual void call(int, int) const = 0;

  };


   void foo(int a, int b, const foo_callback& func);


This is the C++98 way; it is very verbose for the caller, but avoids any
allocation for the caller, and can be passed efficiently downstream.


I'm proposing a fourth way, std::function_ref.  It acts like
std::function, but only refers to a function; it does not capture the
value.  It is the responsibility of the caller to ensure the function is
alive.


   void foo(int a, int b, std::function_ref<void (int a, int b)> func);


Here's a sample program:


void sum_and_report(int a, int b, function_ref<void (int sum)> report) {
   report(a + b);
}

int main(int ac, char** av) {
   int sum;
   sum_and_report(2, 2, [&] (int result) {
     sum = result;
   });
   return sum == 4 ? 0 : 1;
}

and here's an example implementation:


template <typename Ret, typename... Args>
class function_ref<Ret (Args...)> {
   using thunk_type = Ret (*)(const function_ref*, Args...);
   const void* _lambda;
   thunk_type _thunk;
public:
   template <typename Lambda>
   function_ref(const Lambda& lambda)
       : _lambda(&lambda) {
     _thunk = thunk_type([] (const function_ref* zis, Args... args) {
       auto lambda = static_cast<const Lambda*>(zis->_lambda);
       return (*lambda)(std::move(args)...);
     });
   }
   Ret operator()(Args... args) const {
     return _thunk(this, std::move(args)...);
   }
};


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.

On Fri, Dec 9, 2016 at 8:19 AM, Avi Kivity <avi@scylladb.com> wrote:
>
> I'm proposing a fourth way, std::function_ref.  It acts like std::function,
> but only refers to a function; it does not capture the value.  It is the
> responsibility of the caller to ensure the function is alive.
>
>
>   void foo(int a, int b, std::function_ref<void (int a, int b)> func);

See also http://blog.miator.net/post/141640032964/take-callable-by-signature-rargs
, with a few changes on top of the LLVM function_ref.  Not proposed
yet, but I think it's a good idea.

--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
___________________________________________________
4BSD -- http://blog.miator.net/

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I thought that std::function combined with std::reference_wrapper would
already handle this case. Where does it fall short?

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<div dir=3D"ltr">I thought that std::function combined with std::reference_=
wrapper would already handle this case. Where does it fall short?<br></div>

<p></p>

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It allocates (perhaps conditionally in some implementations).

On Dec 9, 2016 18:13, "D. B." <db0451@gmail.com> wrote:

I thought that std::function combined with std::reference_wrapper would
already handle this case. Where does it fall short?

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<https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CACGiwhFkvr%2BT2EMK_GdrmXZWa6JehgD%2BmNf7HqXzOrckJ%2B3uMw%40mail.gmail.com?utm_medium=email&utm_source=footer>
..

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<div dir=3D"auto"><div>It allocates (perhaps conditionally in some implemen=
tations).<br><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On D=
ec 9, 2016 18:13, &quot;D. B.&quot; &lt;<a href=3D"mailto:db0451@gmail.com"=
>db0451@gmail.com</a>&gt; wrote:<br type=3D"attribution"><blockquote class=
=3D"quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-le=
ft:1ex"><div dir=3D"ltr">I thought that std::function combined with std::re=
ference_wrapper would already handle this case. Where does it fall short?<b=
r></div><div class=3D"quoted-text">

<p></p>

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</blockquote></div><br></div></div></div>

<p></p>

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Yes, llvm function_ref and mine are very similar. So it looks like there's
a case for standardization.

On Dec 9, 2016 17:36, "Zhihao Yuan" <zy@miator.net> wrote:

> On Fri, Dec 9, 2016 at 8:19 AM, Avi Kivity <avi@scylladb.com> wrote:
> >
> > I'm proposing a fourth way, std::function_ref.  It acts like
> std::function,
> > but only refers to a function; it does not capture the value.  It is the
> > responsibility of the caller to ensure the function is alive.
> >
> >
> >   void foo(int a, int b, std::function_ref<void (int a, int b)> func);
>
> See also http://blog.miator.net/post/141640032964/take-callable-by-
> signature-rargs
> , with a few changes on top of the LLVM function_ref.  Not proposed
> yet, but I think it's a good idea.
>
> --
> Zhihao Yuan, ID lichray
> The best way to predict the future is to invent it.
> ___________________________________________________
> 4BSD -- http://blog.miator.net/
>
> --
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>

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<div dir=3D"auto">Yes, llvm function_ref and mine are very similar. So it l=
ooks like there&#39;s a case for standardization.</div><div class=3D"gmail_=
extra"><br><div class=3D"gmail_quote">On Dec 9, 2016 17:36, &quot;Zhihao Yu=
an&quot; &lt;<a href=3D"mailto:zy@miator.net">zy@miator.net</a>&gt; wrote:<=
br type=3D"attribution"><blockquote class=3D"gmail_quote" style=3D"margin:0=
 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On Fri, Dec 9, 2016 =
at 8:19 AM, Avi Kivity &lt;<a href=3D"mailto:avi@scylladb.com">avi@scylladb=
..com</a>&gt; wrote:<br>
&gt;<br>
&gt; I&#39;m proposing a fourth way, std::function_ref.=C2=A0 It acts like =
std::function,<br>
&gt; but only refers to a function; it does not capture the value.=C2=A0 It=
 is the<br>
&gt; responsibility of the caller to ensure the function is alive.<br>
&gt;<br>
&gt;<br>
&gt;=C2=A0 =C2=A0void foo(int a, int b, std::function_ref&lt;void (int a, i=
nt b)&gt; func);<br>
<br>
See also <a href=3D"http://blog.miator.net/post/141640032964/take-callable-=
by-signature-rargs" rel=3D"noreferrer" target=3D"_blank">http://blog.miator=
..net/post/<wbr>141640032964/take-callable-by-<wbr>signature-rargs</a><br>
, with a few changes on top of the LLVM function_ref.=C2=A0 Not proposed<br=
>
yet, but I think it&#39;s a good idea.<br>
<br>
--<br>
Zhihao Yuan, ID lichray<br>
The best way to predict the future is to invent it.<br>
______________________________<wbr>_____________________<br>
4BSD -- <a href=3D"http://blog.miator.net/" rel=3D"noreferrer" target=3D"_b=
lank">http://blog.miator.net/</a><br>
<br>
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</blockquote></div></div>

<p></p>

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On Friday, December 9, 2016 at 11:13:06 AM UTC-5, D. B. wrote:
>
> I thought that std::function combined with std::reference_wrapper would
> already handle this case. Where does it fall short?
>

1. There's no way to *prevent* someone from passing something that *isn't*
bound in `reference_wrapper`.

2. The combination of `function` with `reference_wrapper` is not
particularly obvious. As such, it will primarily be used by experts, rather
than lay C++ users.

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<div dir=3D"ltr">On Friday, December 9, 2016 at 11:13:06 AM UTC-5, D. B. wr=
ote:<blockquote class=3D"gmail_quote" style=3D"margin: 0;margin-left: 0.8ex=
;border-left: 1px #ccc solid;padding-left: 1ex;"><div dir=3D"ltr">I thought=
 that std::function combined with std::reference_wrapper would already hand=
le this case. Where does it fall short?<br></div></blockquote><div><br>1. T=
here&#39;s no way to <i>prevent</i> someone from passing something that <i>=
isn&#39;t</i> bound in `reference_wrapper`.<br><br>2. The combination of `f=
unction` with `reference_wrapper` is not particularly obvious. As such, it =
will primarily be used by experts, rather than lay C++ users.<br></div></di=
v>

<p></p>

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On Friday, December 9, 2016 at 11:20:11 AM UTC-5, Avi Kivity wrote:
>
> It allocates (perhaps conditionally in some implementations).
>

From the standard:

>  shall not throw exceptions if f =E2=80=99s target is a callable object p=
assed=20
via reference_wrapper or a function pointer.

So it's not allowed to allocate memory.

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<div dir=3D"ltr">On Friday, December 9, 2016 at 11:20:11 AM UTC-5, Avi Kivi=
ty wrote:<blockquote class=3D"gmail_quote" style=3D"margin: 0;margin-left: =
0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;"><div dir=3D"auto"><di=
v>It allocates (perhaps conditionally in some implementations).<br></div></=
div></blockquote><div><br>From the standard:<br><br>&gt;=C2=A0 shall not th=
row exceptions if f =E2=80=99s target is a callable object passed via refer=
ence_wrapper or a function pointer.<br><br>So it&#39;s not allowed to alloc=
ate memory.</div><br></div>

<p></p>

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On 12/09/2016 06:33 PM, Nicol Bolas wrote:
> On Friday, December 9, 2016 at 11:20:11 AM UTC-5, Avi Kivity wrote:
>
>     It allocates (perhaps conditionally in some implementations).
>
>
> From the standard:
>
> >  shall not throw exceptions if f =E2=80=99s target is a callable object=
=20
> passed via reference_wrapper or a function pointer.
>
> So it's not allowed to allocate memory.
>

Interesting.  That makes the proposal less compelling.

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    <div class=3D"moz-cite-prefix">On 12/09/2016 06:33 PM, Nicol Bolas
      wrote:<br>
    </div>
    <blockquote
      cite=3D"mid:bfbd9c76-0adf-4922-a36d-71233d803235@isocpp.org"
      type=3D"cite">
      <div dir=3D"ltr">On Friday, December 9, 2016 at 11:20:11 AM UTC-5,
        Avi Kivity wrote:
        <blockquote class=3D"gmail_quote" style=3D"margin: 0;margin-left:
          0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;">
          <div dir=3D"auto">
            <div>It allocates (perhaps conditionally in some
              implementations).<br>
            </div>
          </div>
        </blockquote>
        <div><br>
          From the standard:<br>
          <br>
          &gt;=C2=A0 shall not throw exceptions if f =E2=80=99s target is a=
 callable
          object passed via reference_wrapper or a function pointer.<br>
          <br>
          So it's not allowed to allocate memory.</div>
        <br>
      </div>
    </blockquote>
    <br>
    Interesting.=C2=A0 That makes the proposal less compelling.<br>
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.

On Fri, Dec 9, 2016 at 4:45 PM, Avi Kivity <avi@scylladb.com> wrote:
>
> Interesting.  That makes the proposal less compelling.

This is arguable.  function_ref as a vocabulary type, embeds the
meaning of non-owning into the type itself rather than ask the
caller side to choose, IMO is semantically independent,
regardless of lots of other advantages in terms of compilation
time, codegen, etc.

--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
___________________________________________________
4BSD -- http://blog.miator.net/

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.

On 12/10/2016 08:13 AM, Zhihao Yuan wrote:
> On Fri, Dec 9, 2016 at 4:45 PM, Avi Kivity <avi@scylladb.com> wrote:
>> Interesting.  That makes the proposal less compelling.
> This is arguable.  function_ref as a vocabulary type, embeds the
> meaning of non-owning into the type itself rather than ask the
> caller side to choose, IMO is semantically independent,
> regardless of lots of other advantages in terms of compilation
> time, codegen, etc.
>

I'm not saying the proposal is worthless, just that it is less
compelling.  It still has advantages over function_ref, but they are
smaller than what I thought they were.

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.