220 8329 <52C455A7.5090901@gmail.com> article
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From: Matthew Woehlke <mwoehlke.floss@gmail.com>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: Re: Re: Proposal: Default swap operation
Date: Wed, 01 Jan 2014 12:51:35 -0500
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On 2014-01-01 11:07, Bo Persson wrote:
> Matthew Woehlke skrev 2014-01-01 16:26:
>> On 2013-12-31 21:05, fmatthew5876@gmail.com wrote:
>>> On Tuesday, December 31, 2013 8:53:46 PM UTC-5, Billy O'Neal wrote:
>>>> The other issue here is that "using std::swap" would require including
>>>> <algorithm>...
>>>
>>> Implementing swap yourself in this way requires the same.
>>
>> I think the problem here is that the *header* would need to include it.
>>
>> Related to that note, I wonder if we should allow e.g.:
>>
>> foo.h
>> -----
>> class Foo
>> {
>>    ...
>>    ~Foo();
>>    ...
>> }
>>
>> foo.cpp
>> -------
>>
>> Foo::~Foo() = default;
>>
>>
>> IOW, allow uses of '= default' after declaration, to allow the compiler
>> to emit the default implementation at the source level rather than the
>> header level. This would be great for classes where the default
>> implementation would require a complete declaration of a type that
>> otherwise could be forward declared. (Especially thinking of
>> 'unique_ptr<Private>', for which the dtor needs the definition of
>> Private, but otherwise Private could be forward declared.)
>>
>> In this case, this would solve needing <algorithm> in the header.
>>
>
> But wouldn't this instead cause trouble for <type_traits>, like
> is_trivially_destructible<Foo> ?

I don't know. From a conceptual standpoint, in practice, I don't think 
it should; a default dtor defined in the .cpp would necessarily be 
defined as not "trivial". (In practice, you'd be doing this because one 
of the members has a dtor that involves knowledge of the definition of 
some class/struct - in the original example, in order to 'delete' it - 
which IIUC means you're dealing with a non-trivial dtor regardless. IOW, 
in practice, if you are defining a default dtor at the .cpp level it is 
because it is already non-trivial, so forcing it to be considered 
non-trivial because it is defined in the .cpp is no loss.)

I don't know enough about the implementation of 
is_trivially_destructible to know about that, but I would think it 
wouldn't be affected.

IOW, this:

Foo::~Foo() = default

....should be exactly equivalent to this:

Foo::~Foo() {}

....which is to say that the effect is just that the compiler generates 
the body, but it is otherwise semantically equivalent to if the 
programmer had provided a definition.

(The dtor is perhaps not the best example, as the '= default' body is I 
think the same as '{}' in all cases. It's more interesting for e.g. copy 
ctor, which would have a non-trivial body. Keeping in mind that the 
proposal is for *any* default-able member to be "defined" at the .cpp 
level.)

Come to think of it, I'm pretty sure I've run into exactly this before, 
i.e. having to define a "custom" ctor when I really want the default 
just because use of forward declarations means the compiler lacks 
sufficient information to compiler the default definition at the header 
level.

-- 
Matthew

-- 

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