Hmm

I think that what a 'reduce' method is supposed to do is a very common
case. std:;multiset can handle several elements with the same key value, so
it is quite straightforward to have a method in-place which will take care
of this. Implementing this behaviour each time in your code is anyway a
possibility to make a mistake.
Moreover, when 'reduce' is a method, there is a possibility to make its
realization more efficient within the lapse of time (for example, when an
internal structure is changed somehow).
Cause equal_range requires a lot of excessive comparison operations which
might be quite inefficient.
For example, if std::multiset consists of N elements within the same key
value (and N is quite big: 1000, 10000, e.t.c.), your code will require
1000 comparison operations to just delete a couple.

вт, 11 февр. 2025 г. в 17:12, Jonathan Wakely <cxx_at_[hidden]>:

>
>
> On Tue, 11 Feb 2025 at 11:27, Александр Поваляев <apovalyaev_at_[hidden]>
> wrote:
>
>> Hi there,
>>
>> *[My comments below in bold] *
>>
>> вт, 11 февр. 2025 г. в 13:04, Jonathan Wakely <cxx_at_[hidden]>:
>>
>>>
>>>
>>> On Tue, 11 Feb 2025 at 08:30, Александр Поваляев via Std-Proposals <
>>> std-proposals_at_[hidden]> wrote:
>>>
>>>> A Proposal to Add a New Multiset container Method to the Standard
>>>> Library Technical Report
>>>>
>>>> I. Motivation
>>>>
>>>> The idea of a new method is quite straightforward. While std::multiset
>>>> is a constant member of Standard Library Containers
>>>> list, it is quite specific. While being a kind of "extension" to
>>>> std::set, std::multiset exports the same list of methods
>>>> as std::set does.
>>>>
>>>> However, there is an internal structure which makes these two
>>>> containers behave differently. std::multiset provides a tool
>>>> to add a countable number of the elements within the same key value.
>>>> So, 'count' method returns the number of elements
>>>> matching a specific key. For std::set 'count' method can return 0 and 1
>>>> only. Whereas std::multiset 'count' method can
>>>> return 0, 1, ... N corresponding the number of the elements within the
>>>> same key value.
>>>>
>>>> Each time 'insert' method is called:
>>>> * it sets an internal counter to 1 in case of std::set
>>>> * it INCREASES an internal counter in case of std::multiset
>>>>
>>>
>>> There is no such internal counter in reality. The container doesn't need
>>> to keep such a count for each key value (it would just waste memory).
>>>
>>> When you call count, it doesn't return an internal counter, it counts
>>> how many elements match the key.
>>>
>>>
>>>>
>>>> So, here is the case when these two containers behave differently while
>>>> providing right the same interface method.
>>>> It is because of the different in internal containers structure.
>>>>
>>>
>>> They don't have any different structure.
>>>
>>>
>>>>
>>>> The same is true while an element is being erased by means of 'erase'
>>>> method:
>>>> * it sets an internal counter to 0 in case of std::set
>>>> * it DOES NOT DECREASE an internal counter in case of std::multiset -
>>>> it just erased all the elements within the same
>>>> key value and sets and internal counter to 0
>>>>
>>>> Here we go. Compared to the existing standard container std::set,
>>>> std::multiset can offer an advantage making its interface
>>>> more universal:
>>>>
>>>> * a method which acts right the opposite way as 'insert' does
>>>> * a method which can be used to decrease the number of elements with
>>>> the same key value in more discrete. robust and more fine-grained.
>>>>
>>>
>>> You can use std::erase_if to do this, or something like:
>>>
>>> template<typename T, typename Cmp, typename Alloc>
>>> std::size_t
>>> reduce(std::multiset<T, Cmp, Alloc>& s, const T& key, std::size_t count
>>> = -1zu)
>>> {
>>> std::size_t n = 0;
>>> auto [lo,hi] = s.equal_range(key);
>>> auto last = lo;
>>> while (last != hi && std::cmp_less(n, count))
>>> {
>>> ++last;
>>> ++n;
>>> }
>>> s.erase(lo, last);
>>> return n;
>>> }
>>>
>> [Aleksandr] It looks a little bit tricky to implement such an easy
>> operation (just to remove a particular number of elements with the same key
>> value).
>> There is a well-known "common rule" that it is good to
>> hold any class method in bounds of 1 - 15 lines.
>> Without such kind of universality (a single "reduce"
>> method), it will be much harder to follow the rule.
>>
>
> Sorry, I don't understand this. You add the function above to your code,
> now you can call it in one line. The same as if it was a new member
> function of multiset.
>
>
>
>> Erasing elements is not about STL iterators, but more
>> about STL containers functionality.
>> So, introducing such a std::multiset interface
>> extension can be considered as one more step towards STL containers
>> universality.
>>
>>>
>>> It doesn't need to be a member function.
>>>
>>>
>>>
>>>>
>>>> II. Impact On the Standard
>>>>
>>>> This proposal is a pure std::multiset container interface extension.
>>>> While it does not require any change in the core language, it might
>>>> benefit from providing more universal way to do the simple things. It
>>>> allows to express the same logic in less value of C++ code.
>>>>
>>>> III. Design Decisions
>>>>
>>>> This proposal is about adding new method 'reduce' to std::multiset
>>>> container interface.
>>>>
>>>> std::multiset<Key,Compare,Allocator>::reduce
>>>>
>>>> C++ Containers library std::multiset
>>>>
>>>> (1) size_type reduce( const Key& key );
>>>>
>>>> (2)
>>>> template< class K >
>>>> size_type reduce( K&& x );
>>>>
>>>> (3) size_type reduce( const Key& key, size_type count );
>>>>
>>>> (4)
>>>> template< class K >
>>>> size_type reduce( K&& x, size_type count );
>>>>
>>>> This method acts in right the opposite way to how method 'insert' does.
>>>> Method 'reduce' decreases the number of std::multiset elements with
>>>> the same key value. And while being supplied within the second argument
>>>> 'size_type count', method 'reduce' decreases the number of std::multiset
>>>> elements with the same key value but several times.
>>>>
>>>> So, this innovation implies several changes to standard template
>>>> library header file <set>.
>>>>
>>>> VI References
>>>>
>>>> [1] Bjarne Stroustrup, The C++ Programming Language 3rd edition, 1997,
>>>> Addison Wesley ISBN 0-201-88954-4 p496
>>>> [2] Matthew H. Austern, Generic Programming and the STL, 1998, Addison
>>>> Wesley ISBN 0-201-30956-4 pp59-67
>>>> [3] Nicolai M. Josuttis, The C++ Standard Library - A Tutorial and
>>>> Reference, 1999, Addison Wesley ISBN 0-201-37926-0 pp218-221
>>>> вт, 11 февр. 2025 г. в 11:25, Александр Поваляев <apovalyaev_at_[hidden]
>>>> >:
>>>>
>>>>> I. Motivation
>>>>>
>>>>> The idea of a new method is quite straightforward. While std::multiset
>>>>> is a constant member of Standard Library Containers
>>>>> list, it is quite specific. While being a kind of "extension" to
>>>>> std::set, std::multiset exports the same list of methods
>>>>> as std::set does.
>>>>>
>>>>> However, there is an internal structure which makes these two
>>>>> containers behave differently. std::multiset provides a tool
>>>>> to add a countable number of the elements within the same key value.
>>>>> So, 'count' method returns the number of elements
>>>>> matching a specific key. For std::set 'count' method can return 0 and
>>>>> 1 only. Whereas std::multiset 'count' method can
>>>>> return 0, 1, ... N corresponding the number of the elements within the
>>>>> same key value.
>>>>>
>>>>> Each time 'insert' method is called:
>>>>> * it sets an internal counter to 1 in case of std::set
>>>>> * it INCREASES an internal counter in case of std::multiset
>>>>>
>>>>> So, here is the case when these two containers behave differently
>>>>> while providing right the same interface method.
>>>>> It is because of the different in internal containers structure.
>>>>>
>>>>> The same is true while an element is being erased by means of 'erase'
>>>>> method:
>>>>> * it sets an internal counter to 0 in case of std::set
>>>>> * it DOES NOT DECREASE an internal counter in case of std::multiset -
>>>>> it just erased all the elements within the same
>>>>> key value and sets and internal counter to 0
>>>>>
>>>>> Here we go. Compared to the existing standard container std::set,
>>>>> std::multiset can offer an advantage making its interface
>>>>> more universal:
>>>>>
>>>>> * a method which acts right the opposite way as 'insert' does
>>>>> * a method which can be used to decrease the number of elements with
>>>>> the same key value in more discrete. robust and more fine-grained.
>>>>>
>>>>> II. Impact On the Standard
>>>>>
>>>>> This proposal is a pure std::multiset container interface extension.
>>>>> While it does not require any change in the core language, it might
>>>>> benefit from providing more universal way to do the simple things. It
>>>>> allows to express the same logic in less value of C++ code.
>>>>>
>>>>> III. Design Decisions
>>>>>
>>>>> This proposal is about adding new method 'reduce' to std::multiset
>>>>> container interface.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> *std::multiset<Key,Compare,Allocator>::reduceC++ Containers library
>>>>> std::multiset (1) size_type reduce( const Key& key );(2) template< class K
>>>>> >size_type reduce( K&& x );(3) size_type reduce( const Key& key, size_type
>>>>> count );(4)template< class K >size_type reduce( K&& x, size_type count );*
>>>>>
>>>>> This method acts in right the opposite way to how method 'insert'
>>>>> does. Method 'reduce' decreases the number of std::multiset elements with
>>>>> the same key value. And while being supplied within the second
>>>>> argument 'size_type count', method 'reduce' decreases the number of
>>>>> std::multiset
>>>>> elements with the same key value but several times.
>>>>>
>>>>> So, this innovation implies several changes to the standard template
>>>>> library header file <set>.
>>>>>
>>>>> VI References
>>>>>
>>>>> [1] Bjarne Stroustrup, The C++ Programming Language 3rd edition, 1997,
>>>>> Addison Wesley ISBN 0-201-88954-4 p496
>>>>> [2] Matthew H. Austern, Generic Programming and the STL, 1998, Addison
>>>>> Wesley ISBN 0-201-30956-4 pp59-67
>>>>> [3] Nicolai M. Josuttis, The C++ Standard Library - A Tutorial and
>>>>> Reference, 1999, Addison Wesley ISBN 0-201-37926-0 pp218-221
>>>>>
>>>> --
>>>> Std-Proposals mailing list
>>>> Std-Proposals_at_[hidden]
>>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>>
>>>