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Will the constexpr_if feature allow specializing data members in class
templates? So instead of having to specialize class templates, or having to
derive from std::conditional_t, one could do a constexpr_if at class scope
to switch between different implementations (enabling logging/caching
etc.), or different storage containers (deque/vector/list)


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<div dir=3D"ltr">Will the constexpr_if feature allow specializing data memb=
ers in class templates? So instead of having to specialize class templates,=
 or having to derive from std::conditional_t, one could do a constexpr_if a=
t class scope to switch between different implementations (enabling logging=
/caching etc.), or different storage containers (deque/vector/list)<div><br=
></div><div><br></div></div>

<p></p>

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On Thu, Nov 12, 2015 at 1:53 PM, <rhalbersma@gmail.com> wrote:

> Will the constexpr_if feature allow specializing data members in class
> templates? So instead of having to specialize class templates, or having to
> derive from std::conditional_t, one could do a constexpr_if at class scope
> to switch between different implementations (enabling logging/caching
> etc.), or different storage containers (deque/vector/list)
>

No. It's not permitted anywhere a non-constexpr if is not permitted.

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On T=
hu, Nov 12, 2015 at 1:53 PM,  <span dir=3D"ltr">&lt;<a href=3D"mailto:rhalb=
ersma@gmail.com" target=3D"_blank">rhalbersma@gmail.com</a>&gt;</span> wrot=
e:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-l=
eft:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr">Will the constexpr_if=
 feature allow specializing data members in class templates? So instead of =
having to specialize class templates, or having to derive from std::conditi=
onal_t, one could do a constexpr_if at class scope to switch between differ=
ent implementations (enabling logging/caching etc.), or different storage c=
ontainers (deque/vector/list)</div></blockquote><div><br></div><div>No. It&=
#39;s not permitted anywhere a non-constexpr if is not permitted.</div></di=
v></div></div>

<p></p>

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On Thursday 12 November 2015 13:53:46 rhalbersma@gmail.com wrote:
> Will the constexpr_if feature allow specializing data members in class
> templates? So instead of having to specialize class templates, or having to
> derive from std::conditional_t, one could do a constexpr_if at class scope
> to switch between different implementations (enabling logging/caching
> etc.), or different storage containers (deque/vector/list)

P0128 (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0128r0.html)
does not show any inclination towards constexpr_if being usable outside of a
function, but N3329 did. In fact, it said "A static if declaration can appear
wherever a declaration or a statement is legal."

If the implementation of constexpr_if resembles that of what N3329 proposed,
yes, you could use it to decide on members.

But that's not a guarantee. The biggest problem with static if was that it was
overly broad, so constexpr_if shrinks its scope.

--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358

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