One would probably use a multiset to store distinct objects per key (otherwise one would probably map the key to an int).   Common operations are to get all objects with a key or to remove all objects with a key.   When would one want to delete some objects with a key, but not all, without specifying which objects?   Is there any practical use case?   -----Ursprüngliche Nachricht----- Von:Александр Поваляев via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Di 11.02.2025 17:20 Betreff:Re: [std-proposals] A Proposal to Add a New Multiset container Method to the Standard Library Technical Report An:Jonathan Wakely <cxx_at_[hidden]>; std-proposals_at_[hidden]; CC:Александр Поваляев <apovalyaev_at_[hidden]>; 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] <mailto:cxx_at_[hidden]> >:   On Tue, 11 Feb 2025 at 11:27, Александр Поваляев <apovalyaev_at_[hidden] <mailto:apovalyaev_at_[hidden]> > wrote: Hi there,  [My comments below in bold]  вт, 11 февр. 2025 г. в 13:04, Jonathan Wakely <cxx_at_[hidden] <mailto:cxx_at_[hidden]> >:   On Tue, 11 Feb 2025 at 08:30, Александр Поваляев via Std-Proposals <std-proposals_at_[hidden] <mailto: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] <mailto: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>::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 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] <mailto:Std-Proposals_at_[hidden]> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals