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
>>>
>>