[ARROW-486] [C++] arrow::io::MemoryMappedFile can't be casted to arrow::io::FileInterface

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.2.0
• Fix Version/s: 0.2.0
• Component/s: C++
• Environment: Linux
• External issue URL: https://github.com/apache/arrow/issues/16129
• Assignee: Kouhei Sutou
• Reporter: Kouhei Sutou
• Created: 15/Jan/17 06:49
• Updated: 11/Jan/23 07:09
• Resolved: 15/Jan/17 17:19

Description

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.