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From: "Johannes Schaub (litb)" <schaub-johannes@web.de>
Newsgroups: comp.std.c++
Subject: Re: Why is std::array an aggregate?
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Date: Mon, 21 Dec 2009 19:09:08 CST
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Johannes Schaub (litb) wrote:

> James Kanze wrote:
> 
>> On Dec 18, 9:00 pm, Scott Meyers <NeverR...@aristeia.com> wrote:
>>> In TR1, std::tr1::array needed to be an aggregate so that it
>>> could be brace-initialized.  To offer the same capability for
>>> C++0x's std::array, the class could simply declare a
>>> constructor taking a std::initializer_list.  So why is
>>> std::array an aggregate?  Making it a non-aggregate would
>>> permit e.g., giving it a constructor taking a pair of
>>> iterators.
>> 
>> Presumably, so that it can be statically initialized, in order
>> to avoid order of initialization problems.
>>
> 
> I believe this can all be done using variadic templates:
> 
> template<typename T, size_t S>
> struct array {
>    template<typename ...U>
>    constexpr array(U&&... u):elems{ u... } { }
> 
>    T elems[S];
> };
> 

Actually, to be a constexpr constructor, all the parameter types must be
taken by value so that they are literal types. But since the goal is static-
initialization, i think this won't matter. One could specialize "array" for
literal types to get back some performance for dynamic initialization cases,
i suspect.

I wonder though about differences in behavior to normal aggregate
initialization - are there any? Thanks!

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