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Having it at the end:

  template <class... MutexTypes>
  class lock_guard {
  public:
    using mutex_type = Mutex;  // If MutexTypes... consists of the
single type Mutex
    explicit lock_guard(MutexTypes&... m);
    lock_guard(MutexTypes&... m, adopt_lock_t);    // ...  };

is inconvenient for class template argument deduction. Namely, it is
impossible to deduce the adopt_lock_t constructor from a declaration;
instead, the last type in MutexTypes will be deduced to adopt_lock_t and a
compilation error will occur.

What if we added a new constructor that would allow putting it at the
beginning?

    lock_guard(adopt_lock_t, MutexTypes&... m);


   - Old code (C++11 and C++14) wouldn't break because in those cases
   MutexTypes would have size 1 and the template argument would be explicitly
   specified.
   - Empty MutexTypes would cause a compilation error, but that can be
   avoided by simply adding language saying that this constructor is not
   defined when sizeof...(MutexTypes) is 0.
   - Deduction appears to work as one would want:
   http://melpon.org/wandbox/permlink/VQjKh7ugZzZnoBqx

Any problems with this idea?



--
*Brian Bi*

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<div dir=3D"ltr"><div><div>Having it at the end:<br>
<pre class=3D"gmail-codeblock">  template &lt;class... MutexTypes&gt;
  class lock_guard {
  public:
    using mutex_type =3D Mutex;  <span class=3D"gmail-comment">// If <span =
class=3D"gmail-tcode_in_codeblock">MutexTypes...</span> consists of the sin=
gle type <span class=3D"gmail-tcode_in_codeblock">Mutex</span>
</span>
    explicit lock_guard(MutexTypes&amp;... m);
    lock_guard(MutexTypes&amp;... m, adopt_lock_t);
<span class=3D"gmail-comment"><span class=3D"gmail-textit">    // ...</span=
>
</span>  };</pre></div>is inconvenient for class template argument deductio=
n. Namely, it is impossible to deduce the adopt_lock_t constructor from a d=
eclaration; instead, the last type in MutexTypes will be deduced to adopt_l=
ock_t and a compilation error will occur.<br><br></div>What if we added a n=
ew constructor that would allow putting it at the beginning?<br><pre class=
=3D"gmail-codeblock">    lock_guard(adopt_lock_t, MutexTypes&amp;... m);</p=
re><ul><li>Old code (C++11 and C++14) wouldn&#39;t break because in those c=
ases MutexTypes would have size 1 and the template argument would be explic=
itly specified.</li><li>Empty MutexTypes would cause a compilation error, b=
ut that can be avoided by simply adding language saying that this construct=
or is not defined when sizeof...(MutexTypes) is 0.</li><li>Deduction appear=
s to work as one would want: <a href=3D"http://melpon.org/wandbox/permlink/=
VQjKh7ugZzZnoBqx">http://melpon.org/wandbox/permlink/VQjKh7ugZzZnoBqx</a></=
li></ul><p>Any problems with this idea?<br></p><br><br clear=3D"all"><div><=
div><div><div><div><br>-- <br><div class=3D"gmail_signature"><div dir=3D"lt=
r"><div><div dir=3D"ltr"><font color=3D"#c0c0c0"><i>Brian Bi</i></font><br>=
<div></div><div></div><div></div></div></div></div></div>
</div></div></div></div></div></div>

<p></p>

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r">https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAMmfjbP%2B=
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