When should a default destructor be explicitly defined in a code module

Question

I notice that the Apache Arrow C++ libraries frequently define non-inline virtual destructors in code modules. Is there guidance for which classes should/should not have an explicitly defined non-inline destructor? For example, RandomAccessFile contains a destructor declaration in arrow/io/interfaces.h:

class ARROW_EXPORT RandomAccessFile : public InputStream, public Seekable {
   ...
   /// Necessary because we hold a std::unique_ptr
   ~RandomAccessFile() override;
   ...
};

and a corresponding definition in arrow/io/interfaces.cc:

RandomAccessFile::~RandomAccessFile() = default;

But BufferReader has no such explicit declaration nor definition. I haven't been able to find any general C++ guidance for when a default destructor should be explicitly declared or defined, or for the tradeoffs between defining them inline or in the code module, and would like to better understand the tradeoffs.

I suspect that this lack of an explicit destructor on BufferReader is causing linker warnings in my usage of Arrow, and I'm wondering if this is the cause. Specifically, I'm seeing:

INFO: From Linking ...:
/usr/bin/ld.gold: warning: while linking bazel-out/k8-fastbuild/bin/...: symbol 'virtual thunk to arrow::io::BufferReader::~BufferReader()' defined in multiple places (possible ODR violation):
  /usr/include/c++/7/bits/shared_ptr_base.h:170 from bazel-out/k8-fastbuild/bin/external/arrow/_objs/arrow/function_internal.pic.o
  external/arrow/cpp/src/arrow/io/memory.h:145 from bazel-out/k8-fastbuild/bin/external/arrow/_objs/parquet/0/schema.pic.o

Most C++ compilers emit the virtual function table in the translation unit that has the first non-inline virtual (or override) function definition. — Eljay, May 09 '22 at 18:30
dupe/related: https://stackoverflow.com/questions/9020372/how-do-i-use-unique-ptr-for-pimpl . — apple apple, May 09 '22 at 18:59
But the code you present is unlikely to cause the warning (since it'd have only 1 definition in .cc file) — apple apple, May 09 '22 at 19:02
Linker error possibly related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94156 — David Minogue, May 10 '22 at 16:22

When should a default destructor be explicitly defined in a code module

0 Answers0