[Dune] [#758] Default implementation of Dune::array is not a drop-in replacement for std::array (Attachment added)

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User who did this - Bård Skaflestad (bska) 

Attached to Project - Dune
Summary - Default implementation of Dune::array is not a drop-in replacement for std::array
Task Type - Bug Report
Category - Dune Modules
Status - Unconfirmed
Assigned To - 
Operating System - Unspecified / All
Severity - Low
Priority - Normal
Reported Version - SVN (pre2.0)
Due in Version - Undecided
Due Date - Undecided
Details - Guys,

I'm very sorry to be bringing up Dune::array once more.  I know it's been extensively debated (e.g., in FS438) and you probably do not want to rehash the arguments.  Still, if I may, I'd like to point out that the current default implementation of Dune::array (i.e., in the !HAVE_ARRAY case) is not a drop-in replacement for std::array.

Specifically, the (draft) standard for C++0x states (in chapter 23.3.1, point 2) that std::array is an "aggregate" (8.5.1) that can be initialized with the syntax

    array<T,N> a = { initializer-list };

Moreover, an "aggregate" (8.5.1) is 

    ... an array or a class (Clause 9) with no user-provided constructors (12.1), no private or
    protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

For exposition the standard uses the name 'elems' for the internal array representation in std::array.  The actual name, however, is undefined (and the GNU libstc++ implementation uses the name '_M_instance').  The default implementation of Dune::array includes a constructor as well as protected non-static data members (the 'a' array).  Consequently, the following statement

    Dune::array<int,3> a = { {1, 2, 3} };

fails to compile in the !HAVE_ARRAY case.

I have attached a test program (initialize-array.cpp) which demonstrates the problem.  Here, class 'A' is a stripped-down version of the default Dune::array class.  For the purpose of this particular test, Dune::array corresponds to the INCLUDE_CTOR && HIDE_DATA case.  The program builds (and runs) when compiled with GCC 4.1 (which does not provide <array>) and GCC 4.4 (which does provide <array>, at least in -std=c++0x mode).  For kicks, you may define HAVE_ARRAY (on implementations which provide <array>) to demonstrate the std::array behaviour.

Finally, I have attached a (trivial) patch (array.hh.diff) which turns Dune::array into drop-in replacement for std::array--at least from the point of view of initialization lists.  As Dune::array is a fundamental class, however, I hesitate to propose this for inclusion in Dune.  I have build- (and run-) tested the patch on slightly outdated versions of dune-common (-r5920), dune-grid (-r6404), and dune-istl (-r1181), but I have not tested any other modules.

I welcome any and all discussion on this topic.

Best regards,
Bård Skaflestad
SINTEF ICT, Applied Mathematics

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