220 5801 <079a64f7-2606-4309-8472-8624032c2f03@isocpp.org> article
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From: David Krauss <potswa@gmail.com>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: Re: Making std::vector<std::unique_ptr<void>> not be
 copy constructible, among other things
Date: Sun, 25 Aug 2013 10:54:17 -0700 (PDT)
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The problem here is simply a library bug, be it the Standard Library or oth=
er. A constructor shouldn't be declared with no available definition. Some =
other places already state "this signature does not participate in overload=
 resolution..." and the principle needs more general application.

If a library defect hasn't been filed, that's the next order of business, b=
ut this may require a paper, too. The fix could be just adding a sentence t=
o the container requirements table.

Concepts simplify introspection a la wellformed() (it's called requires()) =
but it still only applies to immediate context. You don't want the overhead=
 of instantiating a definition during overload resolution, of a function th=
at might never be used.

Implementing SFINAE on a copy constructor is unfortunately harder than it s=
hould be, and I'm not aware of any fix in the pipeline. Two lightweight alt=
ernatives:

1. Implement the constructor in a base, and inherit from another base which=
 is conditionally copy-constructible, so the implicit definition of the der=
ived constructor is conditionally deleted.

2. Use std::conditional to replace the constructor parameter type with a ha=
rmless, unused tag.

--=20

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