Boost.Range and initializer_list

[Boost - Users mailing list]

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

}

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[Boost - Users mailing list]

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 <[hidden email]>:

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

}

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[Boost - Users mailing list]

Robert Jones via Boost-users wrote:
>   for( int i : { 1, 2, 3 } | reversed ) { cout << i << " "; } // Bad

Boost.Range is not relevant here (other than the prvalue problem).
`{...} | ...` is not allowed in C++.
See https://bugs.llvm.org/show_bug.cgi?id=13069

In C++2a, you can use this

    for (auto rng = {...}; auto&& x : rng | ...) {...}

thanks to http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0614r1.html.

Regards,
Michel
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[Boost - Users mailing list]

Right, a full and comprehensive explanation. Many Thanks.

On 17 April 2018 at 12:17, Michel Morin via Boost-users <[hidden email]> wrote:

Robert Jones via Boost-users wrote:
>   for( int i : { 1, 2, 3 } | reversed ) { cout << i << " "; } // Bad

Boost.Range is not relevant here (other than the prvalue problem).
`{...} | ...` is not allowed in C++.
See https://bugs.llvm.org/show_bug.cgi?id=13069

In C++2a, you can use this

    for (auto rng = {...}; auto&& x : rng | ...) {...}

thanks to http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0614r1.html.

Regards,
Michel

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