std::ranges::find_if - no type in std::common_reference

Question

I'm using the SG14 flat_map as a container.

As per a standard map, it takes Key and Value template parameters.

Unlike a standard map, however, it doesn't store std::pair<Key, Value> in a binary search tree, but rather stores the keys and values in two separate containers (additional template arguments which default to std::vector)

template<
    class Key,
    class Mapped,
    class Compare = std::less<Key>,
    class KeyContainer = std::vector<Key>,
    class MappedContainer = std::vector<Mapped>
>
class flat_map

It then defines a number of types as follows:

using key_type = Key;
using mapped_type = Mapped;
using value_type = std::pair<const Key, Mapped>;
using key_compare = Compare;
using const_key_reference = typename KeyContainer::const_reference;
using mapped_reference = typename MappedContainer::reference;
using const_mapped_reference = typename MappedContainer::const_reference;
using reference = std::pair<const_key_reference, mapped_reference>;
using const_reference = std::pair<const_key_reference, const_mapped_reference>;

If I attempt to use std::ranges::find_if on the flat_map, I get an error:

error: no type named ‘type’ in 
    ‘struct std::common_reference<std::pair<const Key&, const Value&>&&, 
                                  std::pair<const Key, Value>&>’
  121 | auto it = std::ranges::find_if(map, [](auto& kv) { return kv.second.name == "foo"; });
      |           ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If I use a non-range find_if, everything "just works"

auto it = std::find_if(map.begin(), map.end(), [](auto& kv) { return kv.second.name == "foo"; });

Why does the std::ranges::find_if not work?

Example on godbolt: https://godbolt.org/z/r93f7qozr

Edit:

@Brian provided an exemplar which successfully compiles - albeit with slight differences to mine - notably my map is const, and I take the lambda argument as a const ref...

This begs the questions:

• Why does the combination of const range and const auto& lambda argument fail to compile, while pasing a mutable range works and taking the lambda argument by value works?
• I believe it would be considered somewhat of an anti-pattern to take the non-range std::find_if algorithm's lambda arguments by value (auto as opposed to const auto&) as this will cause every element to be copied - and therefore using const auto& should be preferred... principle of least surprose means I assumed the same would be the case with std::ranges - is this not the case?

Answer 1

Why does the combination of const range and const auto& lambda argument fail to compile, while pasing a mutable range works and taking the lambda argument by value works?

First, the operator*() of the iterator of flat_map is defined as follows:

reference operator*() const {
  return reference{*kit_, *vit_};
}

And the type of reference is pair, this means that operator*() will return a prvalue of pair, so the parameter type of the lambda cannot be auto&, that is, an lvalue reference, because it cannot bind rvalue.

Second, const flat_map does not model the input_range concept, that is, its iterator does not model input_iterator which requires indirectly_readable which requires common_reference_with<iter_reference_t<In>&&, iter_value_t<In>&>, the former is pair<const int&, const int&>&&, and the latter is pair<const int, int>&, there is no common_reference for the two.

The workaround is to just define common_reference for them, just like P2321 does (which also means that your code is well-formed in C++23):

template<class T1, class T2, class U1, class U2,
         template<class> class TQual, template<class> class UQual>
  requires requires { typename pair<common_reference_t<TQual<T1>, UQual<U1>>,
                                    common_reference_t<TQual<T2>, UQual<U2>>>; }
struct basic_common_reference<pair<T1, T2>, pair<U1, U2>, TQual, UQual> {
  using type = pair<common_reference_t<TQual<T1>, UQual<U1>>,
                    common_reference_t<TQual<T2>, UQual<U2>>>;
};

For details on common_reference, you can refer to this question.