Removing an empty base class can break ABI

Earlier this week Eric Fiselier pointed out something that came as an unpleasant surprise (to me, at least): Even though C++11 deprecated inheritance-from-std::unary_function, and C++14 deprecated inheritance-from-std::iterator, the actual library vendors cannot remove those deprecated relationships from their libraries without taking an ABI break!

This is eerily apropos to Bryce Adelstein Lelbach’s closing keynote at C++Now 2021, in which ABI breakage was a major theme.


C++98 didn’t have auto, so it was useful for “function objects” like std::plus<T> to expose a member typedef result_type, so that you could write nice generic code like

template<class F, class T>
typename F::result_type
apply_over(F f, T x, T y, T z) {
    return f(f(x,y),z);

Also first_argument_type and second_argument_type. Writing these three member typedefs over and over was a bit tedious, so the C++98 STL provided a base class template std::binary_function and mandated that plus (and less and so on) should inherit from that. N1905 [lib.base] was literally this simple:

template<class Arg1, class Arg2, class Result>
struct binary_function {
    typedef Arg1 first_argument_type;
    typedef Arg2 second_argument_type;
    typedef Result result_type;

template<class T>
struct plus : binary_function<T,T,T> {
    T operator()(const T& x, const T& y) const
        { return x + y; }

The same pattern occurs with the standard iterator model. Every iterator, if you want it to work with std::iterator_traits, must provide five member typedefs: value_type, difference_type, pointer, reference, and iterator_category. (C++20 finally eliminates the requirement to provide pointer and reference!) Writing these five lines was a bit tedious, so C++98 provided a base class template std::iterator and mandated that the standard iterator adaptors should inherit from it.

template<class Category, class T, class Distance = ptrdiff_t,
    class Pointer = T*, class Reference = T&>
struct iterator {
    typedef T value_type;
    typedef Distance difference_type;
    typedef Pointer pointer;
    typedef Reference reference;
    typedef Category iterator_category;

template<class Iterator>
class reverse_iterator : public iterator<
    typename iterator_traits<Iterator>::iterator_category,
    typename iterator_traits<Iterator>::value_type,
    typename iterator_traits<Iterator>::difference_type,
    typename iterator_traits<Iterator>::pointer,
    typename iterator_traits<Iterator>::reference>
    Iterator current;
    // and so on

Notice in passing that reverse_iterator suffers from the same addiction to protected members that plagues std::queue and std::insert_iterator.

Anyway, that was the situation in C++98.

C++11 gave us auto and decltype, and also lambdas. Giving a bunch of member typedefs to every function object suddenly seemed like a dumb idea. So C++11 deprecated the std::unary_function and std::binary_function base class templates (and also introduced std::function, which was not a base class), and removed them as base classes of std::plus et al.

Notice that C++11 did not say that std::plus must not inherit from binary_function! As far as I know, standard types may inherit from whatever types they want. Heck, std::vector<int> could inherit from std::regex if it wanted to.

So, vendors continued to make plus inherit from binary_function, since it was mandated in C++98 mode and not actively harmful in C++11 mode.

C++11 preserved std::iterator as-is, since nothing was particularly changing in that area.

A very late-breaking issue, LWG2438, modified C++14 to remove iterator as a base class of reverse_iterator et al; but did not actually deprecate iterator.

Then in C++17 we almost got Concepts, which made it conspicuously awkward that std::iterator was sitting on a great library name. And Ranges was starting to refactor which of those typedefs you even needed. So P0174 “Deprecating Vestigial Library Parts in C++17” deprecated iterator — but unfortunately not speedily enough for Concepts, which is how we got stuck with the sesquipedalian std::input_or_output_iterator in C++20.

Meanwhile, P0090 “Removing result_type, etc.” deprecated std::plus’s result_type, first_argument_type, and second_argument_type member typedefs for C++17, and P0619 “Reviewing Deprecated Facilities of C++17 for C++20” formally removed them in C++20. (Of course vendors are allowed to provide those typedefs even in C++20.)

So the situation in C++20, as I understand it, is:

  • reverse_iterator must provide those five member typedefs somehow, but not necessarily by inheriting from iterator.

  • plus needn’t provide those three member typedefs at all, let alone by inheriting from binary_function. (But it might.)

Now for the ABI break

Pretend we’re a standard library vendor. Our existing implementation looks like this (simplified):

template<class Iterator>
class reverse_iterator : public std::iterator< ~~~ > {
    Iterator current;

template<class T>
class vector {
    struct iterator { T *ptr_; ~~~ };
    using reverse_iterator = std::reverse_iterator<iterator>;
    reverse_iterator rbegin();

Consider some user code like

std::vector<int> v;
auto it = std::make_reverse_iterator(v.rbegin());

it is now a variable of type reverse_iterator<reverse_iterator<vector<int>::iterator>>.

You might think reversing a reverse_iterator should just unwrap it. Maybe. But weird special cases are hard to reason about. If make_reverse_iterator(t) didn’t always return a reverse_iterator<T>, you know I’d be blogging about some weird pitfall caused by that!

The class layout of decltype(it) is:

  • Empty base class iterator<random_access_iterator_tag, T, ptrdiff_t, T*, T&>

  • Member current, which is of type vector<int>::reverse_iterator, i.e. std::reverse_iterator<std::vector<int>::iterator>

The class layout of decltype(it.current) is:

  • Empty base class iterator<random_access_iterator_tag, T, ptrdiff_t, T*, T&> (again!)

  • Member current, which is of type vector<int>::iterator and occupies sizeof(T*) bytes

So it contains two empty base classes, both of type iterator<random_access_iterator_tag, T, ptrdiff_t, T*, T&>. In C++, base-class subobjects are distinct objects in their own right, and two objects of the same type cannot occupy the same address; so the EBO applies to only one of them. (Jonathan Wakely has called this the “empty-base exclusion principle.”) Our reversed reverse-iterator object ends up occupying 16 bytes, not just 8! (Godbolt showing the behavior on libstdc++.)

If you try this on libc++, you’ll find that the original reverse_iterator occupies 16 bytes already, because libc++’s reverse_iterator wraps a pair of iterators instead of just one. LWG2360 is related; and sadly the extra pointer can’t be dropped without… say it with me… breaking ABI. Yes, this is insane.

So, on a sane library (not you libc++) that supports C++11, sizeof(it) is 16 bytes. Now let’s pretend that the library vendor removes that std::iterator base class, as C++14 permits.

template<class Iterator>
class reverse_iterator
#if __cplusplus < 201402L
    : public std::iterator< ~~~ >
    Iterator current;

Well, now the class layout of decltype(it) doesn’t have those empty base classes anymore! So sizeof(it) drops to 8 bytes. And that changes the size of Widget, and therefore the calling convention of make_widget in (Godbolt):

struct Widget {
    std::reverse_iterator<std::reverse_iterator<int*>> rr;
    bool b;
Widget make_widget() {
    return Widget{ {}, false };
      // A trivially copyable 24-byte type is returned on the stack.
      // A trivially copyable 16-byte type is returned in registers, instead.

Any C++11 caller linking against a C++14 make_widget, or vice versa, will expect the Widget result in the wrong place and occupying the wrong number of bytes.

Conclusion: For a library vendor, removing the deprecated empty base class from reverse_iterator counts an ABI break.

UPDATE, 2021-05-08: It’s been brought to my attention that the C++20 Ranges library starts the whole damn cycle over again! Ranges provides a convenience base class

struct view_base {};

which isn’t even a template at all — it’s literally no more than a tag type — but then the CRTP base class view_interface inherits from that, and then every range adaptor inherits from some specialization of view_interface!

template<class Crtp>
struct view_interface : view_base { ~~~ };

template<class V>
class reverse_view : public view_interface<reverse_view<V>> { ~~~ };

The result is that you would get the same bloated class layout from reverse_view as you get from reverse_iterator… except that Ranges actually does the “optimization” where reversing a reverse_view produces the original view again! That is, std::views::reverse is overloaded so that when its argument x is a reverse_view, the result of std::views::reverse(x) is just x.base(). (Godbolt.)

If Ranges is still around 10 years from now, I predict at least a couple papers deprecating inheritance from view_base.

Posted 2021-05-07