arrow

ARROW-486: [C++] Use virtual inheritance for diamond inheritance

arrow::io::ReadWriteFileInterface inheritances arrow::io::FileInterface
as diamond style via:

  * ReadableFileInterface -> InputStream -> FileInterface
  * WriteableFileInterface -> OutputStream -> FileInterface

If we have diamond inheritance, we can't cast subclasses of
arrow::io::ReadWriteFileInterface such as arrow::io::MemoryMappedFile to
arrow::io::FileInterface.

C++:

    #include <arrow/io/file.h>

    int
    main(void)
    {
      std::shared_ptr<arrow::io::MemoryMappedFile> memory_mapped_file;

      std::shared_ptr<arrow::io::FileInterface> file = memory_mapped_file;

      return 0;
    }

Build result:

    a.cc: In function 'int main()':
    a.cc:8:52: error: conversion from 'std::shared_ptr<arrow::io::MemoryMappedFile>' to non-scalar type 'std::shared_ptr<arrow::io::FileInterface>' requested
       std::shared_ptr<arrow::io::FileInterface> file = memory_mapped_file;
                                                        ^~~~~~~~~~~~~~~~~~

We can resolve it by using virtual inheritance.

Author: Kouhei Sutou <kou@clear-code.com>

Closes #282 from kou/use-virtual-inheritance-for-diamond-inheritance and squashes the following commits:

e9d3a0b [Kouhei Sutou] ARROW-486: [C++] Use virtual inheritance for diamond inheritance