P2249R3
Mixed comparisons for smart pointers

1. Changelog

• R3

  • Clarified some design decisions after a review on the LEWG mailing list.

  • Changed the proposed wording for the constraints, in order to exclude types that are derived from unique_ptr or shared_ptr specializations.

• R2

  • Fixes to the proposed wording.

• R1

  • Added some clarifications to the design decisions, following a review on the LEWG mailing list.

  • Made the proposed operators hidden friends.

  • Rebased on top of the latest draft of the Standard.

  • Minor fixes.

• R0

  • First submission.

2. Tony Tables

class Manager
{
    // The Manager owns the Objects, therefore it uses smart pointers.
    // The Manager gives out non-owning raw pointers to clients.
    // A client gives the raw pointer back to the Manager, to tell it
    // to act on that Object; the Manager has then to look up the Object
    // in its storage.

    std::vector<std::unique_ptr<Object>> objects;

public:
    Object* get_object(~~~) const
    {
        return objects[~~~].get();
    }

    void drop_object(Object* input)
    {
        // Must use erase_if and a custom comparison (e.g. a lambda).
        auto isEqual = [input](const std::unique_ptr<Object>& o) {
            return o.get() == input;
        };
        erase_if(objects, input);
    }

    ssize_t index_for_object(Object* input) const
    {
        // Same story.
        // Code like this (predicates, etc.) may get duplicated all over the place
        // where smart pointers are used in containers/algorithms. Surely,
        // centralizing it is good practice, but there’s always the temptation of
        // just writing the one-liner lambda and "moving on" rather than refactoring...
        auto isEqual = [input](const std::unique_ptr<Object>& o) {
            return o.get() == input;
        };
        auto it = std::ranges::find_if(objects, isEqual);
        // etc.
    }
};

class Manager
{





    std::vector<std::unique_ptr<Object>> objects;

public:
    Object* get_object(~~~) const
    {
        return objects[~~~].get();
    }

    void drop_object(Object* input)
    {




        // Just use a value-based algorithm, no need for a predicate!
        erase(objects, input);
    }

    ssize_t index_for_object(Object* input) const
    {




        // Same, just use a value-based algorithm.
        // Unfortunately, the range version does not work as-is because
        // std::equality_comparable_with<std::unique_ptr<Object>, Object*>
        // is not satisfied. See the discussion here.
        auto it = std::ranges::find(objects, input);
        // etc.
    }
};

// Suppose insteat that the Manager needs to use an associative container rather than
// a sequential container (e.g. mapping some data to each object).
// Then, an heterogeneous comparator becomes a necessity -- we can’t possibly
// look up a unique_ptr using another unique_ptr to the same object, especially
// if clients give us non-owning raw pointers to act upon.

// Heterogeneous comparator
template <class T> struct smart_pointer_comparator {
    struct is_transparent {};

    bool operator()(const std::unique_ptr<T>& lhs, const std::unique_ptr<T>& rhs) const
    { return lhs < rhs; }
    bool operator()(const std::unique_ptr<T>& lhs, const T* rhs) const
    { return std::less()(lhs.get(), rhs); }
    bool operator()(const T* lhs, const std::unique_ptr<T>& rhs) const
    { return std::less()(lhs, rhs.get()); }
};

// A sorted associative container with some data
std::map<std::unique_ptr<Object>, Data,
    smart_pointer_comparator<Object>> objects = ~~~;

// Heterogeneous lookup using a raw pointer
object* ptr = ~~~;
auto it = objects.find(ptr);
if (it != objects.end()) { use(it->second); }

// No need for a custom comparator...

















// ... just use the idiomatic std::less<void>
std::map<std::unique_ptr<Object>, Data,
    std:less<>> objects = ~~~;

// Heterogeneous lookup
Object* ptr = ~~~;
auto it = objects.find(ptr);
if (it != objects.end()) { use(it->second); }

// Same, with an unordered associative container.

// Heterogeneous hasher; [util.smartptr.hash] guarantees that both the
// specializations below return the very same value for the same pointer.
template <class T> struct smart_pointer_hasher
{
    struct is_transparent {};
    size_t operator()(const std::unique_ptr<T>& ptr) const {
        // equal by definition to std::hash<T*>(ptr.get()), that is, (*this)(ptr.get())
        return std::hash<std::unique_ptr<T>>()(ptr);
    }
    size_t operator()(T* ptr) const {
        return std::hash<T*>()(ptr);
    }
};

// Heterogeneous equality comparator
template <class T> struct smart_pointer_equal
{
    struct is_transparent {};
    bool operator()(const std::unique_ptr<T>& lhs, const std::unique_ptr<T>& rhs) const
    { return lhs == rhs; }
    bool operator()(const std::unique_ptr<T>& lhs, const T* rhs) const
    { return lhs.get() == rhs; }
    bool operator()(const T* lhs, const std::unique_ptr<T>& rhs) const
    { return lhs == rhs.get(); }
};

std::unordered_map<std::unique_ptr<Object>, Data,
    smart_pointer_hasher<Object>,
    smart_pointer_equal<Object>> objects = ~~~;

// Heterogeneous lookup
Object* ptr = ~~~;
auto it = objects.find(ptr);
if (it != objects.end()) { use(it->second); }

// [P0919R3] does not provide a heterogeneous hasher for smart pointers,
// so a custom one is still needed


template <class T> struct smart_pointer_hasher
{
    struct is_transparent {};
    size_t operator()(const std::unique_ptr<T>& ptr) const {
        // equal by definition to std::hash<T*>(ptr.get()), that is, (*this)(ptr.get())
        return std::hash<std::unique_ptr<T>>()(ptr);
    }
    size_t operator()(T* ptr) const {
        return std::hash<T*>()(ptr);
    }
};

// Custom heterogeneous equality comparator not needed any more











std::unordered_map<std::unique_ptr<Object>, Data,
    smart_pointer_hasher<Object>,
    std::equal_to<>> objects = ~~~;

// Heterogeneous lookup
Object* ptr = ~~~;
auto it = objects.find(ptr);
if (it != objects.end()) { use(it->second); }

3. Motivation and Scope

Smart pointer classes are universally recognized as the idiomatic way to express ownership of a resource (very incomplete list: [Sutter], [Meyers], [R.20]). On the other hand, raw pointers (and references) are supposed to be used as non-owning types to access a resource.

Both smart pointers and raw pointers, as their name says, share a common semantic: representing the address of an object.

This semantic comes with a set of meaningful operations; for instance, asking if two (smart) pointers represent the address of the same object. operator== is used to express this intent.

Indeed, with the owning smart pointer class templates available in the Standard Library (unique_ptr and shared_ptr), one can already use operator== between two smart pointer objects (of the same class). However one cannot use it between a smart pointer and a raw pointer, because the Standard Library is lacking that set of overloads; instead, one has to manually extract the raw pointer out of the smart pointer class:

std::shared_ptr<object> sptr1, sptr2;
object* rawptr;

// Do both pointers refer to the same object?
if (sptr1 == sptr2) { ~~~ }        // WORKS
if (sptr1 == rawptr) { ~~~ }       // ERROR, no such operator
if (sptr1.get() == rawptr) { ~~~ } // WORKS; but why the extra syntax?

This discussion can be easily generalized to the full set of the six relational operations; these operations have already well-established semantics, and are indeed already defined between smart pointers objects (of the same class) or between raw pointers, but they are not supported in mixed scenarios.

Allowing mixed comparisons isn’t merely a "semantic fixup"; the situation where one has to compare smart pointers and raw pointers commonly occurs in practice (the typical use case is outlined in the first example in the § 2 Tony Tables above, where a "manager" object gives non-owning raw pointers to clients, and the clients pass these raw pointers back to the manager, and now the manager needs to do mixed comparisons).

3.1. Associative containers

Moreover, we believe that allowing mixed comparisons is useful in order to streamline heterogeneous comparison in associative containers for smart pointer classes.

The case of an associative container using a unique_ptr as its key type is particularly annoying; one cannot practically ever look up in such a container using another unique_ptr, as that would imply having two unique_ptr objects owning the same object. Instead, the typical lookup is heterogeneous (by raw pointer); this proposal is one step towards making it more convenient to use, because it enables the usage of the standard std::less or std::equal_to.

We however are not addresssing at all the issue of heterogeneous hashing for smart pointers. While likely very useful in general, heterogeneous hashing can be tackled separately by another proposal that builds on top of this one (for instance, by making the std::hash specializations for Standard smart pointers 1) transparent, and 2) able to hash the smart pointer’s pointer_type / element_type* as well as the smart pointer object itself. But more research and field experience is certainly needed.)

4. Impact On The Standard

This proposal is a pure library extension. It proposes changes to an existing header, <memory>, but it does not require changes to any standard classes or functions and it does not require changes to any of the standard requirement tables. The impact is positive: code that was ill-formed before becomes well-formed.

This proposal does not depend on any other library extensions.

This proposal does not require any changes in the core language.

[P0805R2] is vaguely related to this proposal. It proposes to add mixed comparisons between containers of the same type (for instance, to be able to compare a vector<int> with a vector<long>), without resorting to manual calls to algorithms; instead, one can use a comparison operator. A quite verbose call to std::equal(v1.cbegin(), v1.cend(), v2.cbegin(), v2.cend()) can therefore be replaced by a much simpler v1 == v2. In this sense, [P0805R2] matches the spirit of the current proposal, although comparing smart pointers and raw pointer does not require any algorithm, and does not have such a verbose syntax.

5. Design Decisions

5.1. Should unique_ptr have the full set of ordering operators (<, <=, >, >=), or just <=>?

[P1614R2] added support for operator<=> across the Standard Library. Notably, it added operator<=> for unique_ptr, leaving the other four ordering operators (<, <=, >, >=) untouched. On the other hand, when looking at shared_ptr, the same paper replaced these four operators with operator<=>.

We believe that this was done in order to preserve the semantics for the existing operators, which are defined in terms of customization points (notably, common_type; unique_ptr can work in terms of a custom "fancy" pointer type). We are not bound by any pre-existing semantics, so we are just proposing operator<=> for unique_ptr.

What does LEWG(I) think about this?

5.2. Should operators for a smart_pointer<T> accept precisely T* or anything convertible to T*?

Technically speaking, neither: we are proposing to accept anything which is comparable to T*.

The rationale of this proposal is that smart pointers should act like raw pointers when it comes to comparison operators. For instance, raw pointers allow for mixed comparisons if both pointers are convertible to their composite pointer type ([expr.type]):

Base* b = ~~~;
Derived* d = ~~~;

if (b == d) { ~~~ } // OK

Therefore, we want the following to also work:

std::unique_ptr<Base> b = ~~~;
Derived* d = ~~~;

if (b == d) { ~~~ } // OK, with this proposal

As mentioned, the design that we are proposing is actually more general than simply accepting T* or types convertible to T*: we are proposing to accept any data type that can be compared against T*. This may include, for instance, non-owning/observer pointers like observer_ptr (not in the Standard). Such pointer classes are not necessarily implicitly convertible to a raw pointer type. Yet, it makes completely sense to allow for a mixed comparison against them (provided that they also define mixed comparison operators). For instance:

std::unique_ptr<Object> owning_ptr = ~~~;

// non-owning; not in the Standard
observer_ptr<Object> non_owning_ptr(owning_ptr);

if (owning_ptr == non_owning_ptr) { ~~~ }

The last comparison makes semantic sense, and is therefore allowed by this proposal.

Please note that the existing comparison operators for smart pointers already allow for mixed comparisons (between different specializations of the same smart pointer class template). This proposal is therefore not introducing any inconsistency.

5.3. Would these operations make the equality_comparable_with or three_way_comparable_with concepts satisfied between a smart pointer and a raw pointer?

No. Adding the operations would indeed bring the Standard Library’s smart pointer classes "one step closer" to satisfy those concepts, but they would still be unsatisfied because there is no common_reference_t between a smart pointer and a raw pointer.

Changing that is out of scope for the present proposal (and orthogonal to it).

static_assert(equality_comparable_with<unique_ptr<int>, nullptr_t>);  // ERROR (1)
static_assert(equality_comparable_with<shared_ptr<int>, nullptr_t>);  // OK    (2)

static_assert(three_way_comparable_with<unique_ptr<int>, nullptr_t>); // ERROR (3)
static_assert(three_way_comparable_with<shared_ptr<int>, nullptr_t>); // ERROR (4)

An unfortunate consequence of this is that, for the moment being, we will yet not be able to use many range-based algorithms using heterogeneous comparisons:

std::vector<std::unique_ptr<Object>> objects;

Object *ptr = ~~~;

auto it = std::ranges::find(objects, ptr); // ERROR; must use find_if and a predicate

On the other hand, non-range algorithms would work just fine:

auto it = std::find(objects.begin(), objects.end(), ptr); // OK
erase(objects, ptr); // OK

5.4. Should mixed comparisons between different smart pointer classes be allowed?

There are some considerations that in our opinion apply to this case.

The first is whether such an operation makes sense. From the abstract point of view of "comparing the addresses of two objects", the operation is meaningful, even if the addresses are represented by instances of different smart pointer classes. As mentioned before, the currently existing comparison operators of the smart pointer classes implement these semantics.

std::unique_ptr<int> a(new int(123));
std::unique_ptr<int> b(new int(456));

if (a < b)             { ~~~ } // well-defined behavior
if (a.get() < b.get()) { ~~~ } // unspecified behavior [expr.rel/4.3]

On the other hand: the Standard Library smart pointer classes that this proposal deals with are all owning classes. One could therefore reason that there is little utility at allowing a mixed comparison, because it’s likely to be a mistake by the user. In a "ordinary" program, such comparisons would inevitably yield false, because the same object cannot be owned by two smart pointers of different classes (if it is, there is a bug in the program).

There is some semantic leeway here, represented by the fact that the smart pointer classes can hold custom deleters (incl. empty/no-op deleters), aliased pointers (in the case of shared_ptr), as well as as fancy pointer types (in the case of unique_ptr). In principle, one can write a perfectly legal example where the same object is owned by smart pointers of different classes, using custom deleters and/or custom fancy pointer types, and then wants to compare the smart pointers (compare the addresses of the objects owned by them).

In some ways, this is hardly a realistic use case for allowing comparisons between different smart pointer classes. The danger of misuse of such comparisons, again in "ordinary" code, seems be stricly greater than their usefulness, given the unlikelihood of valid use cases -- in the majority of ordinary usages, the comparisons would be meaningless.

We have therefore a tension between the abstract/ideal domain of the operations, and the practical usage. The problem is that trying to solve it via the type system alone isn’t possible. For instance, type-erased deleters (in shared_ptr) make it impossible to know if, given a unique_ptr<X, D> and a shared_ptr<X> object, a comparison between them is meaningless or instead they have somehow been designed to "work together".

A possible conservative solution could be to ban the mixed comparison via an explicit constraint/requirement on the proposed operators. That would however be nothing but a band-aid measure, as it wouldn’t extend to other owning and non-owning smart pointer classes not from the Standard Library (like Boost’s, Qt’s, and so on).

In conclusion: given that

1. although admittely rarely used in practice, the operation is still meaningful;

2. a proper solution is not implementable in the type system; and

3. simply disallowing two smart pointer classes (from the Standard Library) while allowing third-party ones creates a major inconsistency,

in the present proposal we are not going to explicitly ban the comparisons between different smart pointer classes.

Despite this fact, the current version of the proposal still forbids them, although through indirect means: the § 6.2 Proposed wording disallows them due to the requirement clauses, which are currently unsatisfied (see § 5.3 Would these operations make the equality_comparable_with or three_way_comparable_with concepts satisfied between a smart pointer and a raw pointer?) when using the Standard Library smart pointer classes.

Other owning and non-owning smart pointer classes, not from the Standard Library, may or may not end up being comparable to the ones in the Standard Library using the operators that we are proposing, depending on whether they satisfy or not the requirements.

5.5. Should the proposed operators be constexpr?

If [P2273] gets adopted, then the proposed operators for unique_ptr should indeed become constexpr. On the other hand, the ones for shared_ptr shouldn’t, for consistency with the pre-existing ones.

6. Technical Specifications

All the proposed changes are relative to [N4892].

6.1. Feature testing macro

Add to the list in [version.syn]:

#define __cpp_lib_mixed_smart_pointer_comparisons YYYYMML  // also in <memory>

with the value specified as usual (year and month of adoption).

6.2. Proposed wording

6.2.1. unique_ptr

Modify [unique.ptr.single.general] as shown:

    // [unique.ptr.single.mixed.cmp], mixed comparisons
    template<class U>
      requires equality_comparable_with<pointer, U>
        friend bool
        operator==(const unique_ptr& x, const U& y);
    template<class U>
      requires three_way_comparable_with<pointer, U>
      friend compare_three_way_result_t<pointer, U>
        operator<=>(const unique_ptr& x, const U& y);


    // disable copy from lvalue
    unique_ptr(const unique_ptr&) = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
};

Add a new section at the end of the [unique.ptr.single] chapter:

?.?.?.?.? Mixed comparison operators [unique.ptr.single.mixed.cmp]

template<class U>
  requires equality_comparable_with<pointer, U>
  friend bool
    operator==(const unique_ptr& x, const U& y);

1 Constraints: U is not derived from a specialization of unique_ptr, and is_null_pointer_v<U> is false.
2 Returns: x.get() == y.

template<class U>
  requires three_way_comparable_with<pointer, U>
  friend compare_three_way_result_t<pointer, U>
    operator<=>(const unique_ptr& x, const U& y);

3 Constraints: U is not derived from a specialization of unique_ptr, and is_null_pointer_v<U> is false.
4 Returns: compare_three_way()(x.get(), y).

Modify [unique.ptr.runtime.general] as shown:

    // mixed comparisons
    template<class U>
      requires equality_comparable_with<pointer, U>
      friend bool
        operator==(const unique_ptr& x, const U& y);
    template<class U>
      requires three_way_comparable_with<pointer, U>
      friend compare_three_way_result_t<pointer, U>
        operator<=>(const unique_ptr& x, const U& y);


    // disable copy from lvalue
    unique_ptr(const unique_ptr&) = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
};

6.2.2. shared_ptr

Modify [util.smartptr.shared.general] as shown:

    template<class U>
      bool owner_before(const weak_ptr<U>& b) const noexcept;


    // [util.smartptr.shared.mixed.cmp], mixed comparisons
    template<class U>
      requires equality_comparable_with<element_type*, U>
        friend bool operator==(const shared_ptr& a, const U& b);

    template<class U>
      requires three_way_comparable_with<element_type*, U>
      friend compare_three_way_result_t<element_type*, U>
        operator<=>(const shared_ptr& a, const U& b);

};

Insert a new section after [util.smartptr.shared.cmp]:

?.?.?.? Mixed comparison operators [util.smartptr.shared.mixed.cmp]

template<class U>
  requires equality_comparable_with<element_type*, U>
    friend bool operator==(const shared_ptr& a, const U& b);

1 Constraints: U is not derived from a specialization of shared_ptr, and is_null_pointer_v<U> is false.
2 Returns: a.get() == b.

template<class U>
  requires three_way_comparable_with<element_type*, U>
  friend compare_three_way_result_t<element_type*, U>
    operator<=>(const shared_ptr& a, const U& b);

3 Constraints: U is not derived from a specialization of shared_ptr, and is_null_pointer_v<U> is false.
4 Returns: compare_three_way()(a.get(), b).

7. Implementation experience

A working prototype of the changes proposed by this paper, done on top of GCC 11, is available in this GCC branch on GitHub.

8. Acknowledgements

Credits for this idea go to Marc Mutz, who raised the question on the LEWG reflector, receiving a positive feedback.

Thanks to the reviewers of early drafts of this paper on the std-proposals mailing list.

Thanks to KDAB for supporting this work.

All remaining errors are ours and ours only.