Is it true that it is not defined, which elements of several having the same key are selected for removal?   -----Ursprüngliche Nachricht----- Von:Александр Поваляев via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Di 11.02.2025 09:30 Betreff:Re: [std-proposals] A Proposal to Add a New Multiset container Method to the Standard Library Technical Report An:std-proposals_at_[hidden]; CC:Александр Поваляев <apovalyaev_at_[hidden]>; 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 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 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] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals