Is there assign_or_inserter/insert_or_assigner in C++20?

Question

std::inserter suffers from problem that it calls insert that for map is noop if key already exists. There is std::map::insert_or_assign, but I was unable to find an inserter that uses that.

Is there something like this in C++20?

note: I know I can c/p it from somewhere on the SO/internet, I am interested in STL/boost solutions, not c/p implementation from somewhere.

Answer 1

No, there is no corresponding inserter that use insert_or_assign. In fact, the wording explicitly says there are only 3 such functions:

back_­inserter, front_­inserter, and inserter are three functions making the insert iterators out of a container.

Answer 2

There is no insert iterator with insert_or_assign functionality in the C++20 Standard Library

Insert iterators are governed by [insert.iterators], and it is particularly described in [insert.iterators]/1 that the only functions to create iterators

[...] back_­inserter, front_­inserter, and inserter are three functions making the insert iterators out of a container. [...]

are back_­inserter, front_­inserter, and inserter, which, as covered by [back.inserter], [front.inserter] and [inserter], respectively, returns back_insert_iterator, front_insert_iterator and insert_iterator, respectively.

As governed by [back.insert.iter.ops], [front.insert.iter.ops] and [insert.iter.ops], the effect of copy assignment of a value to these iterators is as if calling

// back_insert_iterator
container->push_­back(value);

// front _insert_iterator
container->push_­front(value); 

// insert_iterator
container->insert(iter, value);

respectively. I.e., none of these insert iterators offer the effect of the insert_or_assign member functions of std::map and std::unordered_map (which was introduced by N4713, which makes no mention whatsoever regarding insert iterators in the sense of [insert.iterators]).

Answer 3

There are a few hurdles that make it unlikely to be standardised.

map::insert_or_assign doesn't have a value_type (reference) parameter, which is common across each of insert, push_front and push_back. Remember that operator= only has one parameter, so you'll have to construct a pair to assign to your proxy.

It would be desirable to forward either or both of key and value, how do you distinguish passing const std::pair<const Key, Value> &, std::pair<Key &&, const Value &> etc.

There are only 2 standard containers that have insert_or_assign, map and unordered_map. Every container has insert, and all the sequence containers have one or both of push_front or push_back.