Has always been in there (rB61050f75b13e), I'll replace it in a separate commit.

Buffer is too generic. time_str ?

Any specific reason to use std::size_t instead of size_t ?
time_str_len ?

I would also suggest moving this time-to-string logic to an own function.
Maybe even returning string. Performance hit is neglectable, but from API is more clear.

In the future use unique_ptr.

See other comment about deisgnated initializers.

I don't think you need this. io is the parent namespace, regular symbol resolution in C++ will find ExportSubset for you.

That was the entire idea of putting blender as parent namespace, so you can get rid of using statements.

Its an old code, but using namespace is highly discouraged.

This is confusing.

I do no think you need struct in C++. You should be able to simply Scene *scene.

For the readability never omit parameter names, wrap them into comment instead: Scene * /*scene*/ (and this is where having * next to the type ;)

But then also, why you discard explicitly passed scene and go via the context?

Any specific reason to use MEM_mallocN instead of new ?

See MEM_CXX_CLASS_ALLOC_FUNCS.

Are you up- or down- casting here? Hard to tell because there is no indications in this patch.

You don't need to cast for delete. Unless the destructor is NOT marked as virtual (which it should be).

Designated initializers are part of the C standard, they are not part of C++ standard.

In some compilers and settings they appear to work if you provide all fields in exact order of their declaration. They will not work the same as in C.

I would strongly advice against using them.

This can be changed:

unique_ptr<MyClass*> object(new MyClass());
if (error_happenned) {
  return nullptr;
}
return object.release();

See the inlined comment about designated initialized below.

If you don't sue them you can't have these fields const. Make them non-const and use const qualifier to the variable itself:

const ABCWriterConstructorArgs my_args = Foo();

override implies virtual, so from the language point of view it's file.

For the readability it's arguably better to have explicit virtual anyway. However, some of the tidy warnings considers this bad readability.

Anyway, need to be consistent at least. Here you rely on implicit virtual but few lines above you explicitly mark virtual.

explicit

Unless is absolutely needed mark all single-argument constructors as explicit.

Move extern "C" to headers as much as possible.

Ideally should be in own function, for readability.

triangulated_mesh

Is OK, but not super nice. "Just" iterate the list.

Put tests into the namespace. Discourage using namespace

I un-did the change here, so that this code remains untouched. Will address this comment in a future commit.