Re: [Boost-users] Boost.Range and initializer_list

Subject: Re: [Boost-users] Boost.Range and initializer_list
From: Bart Verhagen (barrie.verhagen_at_[hidden])
Date: 2018-04-17 08:15:46

Hi Robert,

I do not know the specifics of Boost.Range, but I believe what you are
trying to do here is in fact undefined behaviour for initializer_lists.
Reported defect DR1290 (see reference 1) explains the idea behind the
temporary storage of an initializer_list in more detail. Some compilers
(notably Clang) 'exploit' this undefined behaviour in order to further
optimize the usage of initializer_lists. On the other hand, using it to
initialize a vector and using this vector (which is more strict on the
lifetime of its storage) is fully defined and therefore works as expected.

As a result, Boost.Range, which does not necessarily require copy semantics
and therefore uses a reference to the collection (see reference 2), can
only do the right thing while the storage behind the object still exists.

Kindest Regards,

Bart

References:
1. http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1290
2.
https://www.boost.org/doc/libs/1_66_0/libs/range/doc/html/range/concepts/overview.html

2018-04-16 23:06 GMT+02:00 Robert Jones via Boost-users <
boost-users_at_[hidden]>:

> Hi All
>
> Does the Boost.Range library support initializer_list as a range type? I
> thought anything with begin() and end() methods was fair game, but this
> code doesn't work, unless I'm doing it wrong.
>
> #include <iostream>
> #include <vector>
> #include <boost/range/adaptor/reversed.hpp>
>
> int main( )
> {
>   using namespace std;
>   using namespace boost::adaptors;
>
>   vector<int> v { 1, 2, 3 };
>   for( int i : v           | reversed ) { cout << i << " "; } // Ok
>   for( int i : { 1, 2, 3 } | reversed ) { cout << i << " "; } // Bad
> }
>
> Kind Regards
>
> Rob.
>
>
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