One more foolishness of the C++ Standard
in [Visual C]
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From: Ulrich Eckhardt on 21 Jan 2010 11:34 Vladimir Grigoriev wrote: > There is no a great sense to compare some size_t value with an object of > type Person. This comparison has no sense. It is a general situation when > converting some object of a class to an integral type has sense while > converting an integral type to an object has no sense. For this purpose > the explicit reywoard was introduced. The same is valid for operators. > i.e. sometimes a op b has sense while b op a has no sense. I'd say whether switching operands still has sense depends on the operator. For an equality comparison, I expect it to be fully commutative. Similarly for inequality, while e.g. greater behaves slightly different. Are you really implying that a==b makes sense when b==a doesn't? I already voiced my opinion on comparison of a person with an ID: "I wouldn't overload operator== for that." Uli -- C++ FAQ: http://parashift.com/c++-faq-lite Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932
From: Vladimir Grigoriev on 21 Jan 2010 12:26 There are situations when A is a subset of B and in this case you may apply some relation or comparison operator showing that A is a subset of B. However you may not state that B is a subset of A. So a reverse relation is absent and consequently reverse operators also are absent. For example you can define a Point class derived from std::pair. And your Point class may have arithmetic operators. You may to add a std::pair object to a Point object but your may not add a Point object to a std:pair object. Otherwise std::pair class will have a feature which was not supposed to be. The same situation can be with relation operators. Vladimir Grigoriev "Ulrich Eckhardt" <eckhardt(a)satorlaser.com> wrote in message news:5acm27-40c.ln1(a)satorlaser.homedns.org... > Are you really implying that a==b makes sense when b==a doesn't? I already > voiced my opinion on comparison of a person with an ID: "I wouldn't > overload operator== for that." > > Uli > > -- > C++ FAQ: http://parashift.com/c++-faq-lite > > Sator Laser GmbH > Geschaftsfuhrer: Thorsten Focking, Amtsgericht Hamburg HR B62 932
From: Vladimir Grigoriev on 21 Jan 2010 12:34 The problem is the same as with std::min and std::min_element. I do not see any reasonable explanation why these restrictions were adopted. It seems that this shrinks functionality of C++ without any serious reason. For example all that I have heard here relative to std::min is some references to sort algorithms. However I do not see a relation between std::min and sort algorithms.. Vladimir Grigoriev "Ulrich Eckhardt" <eckhardt(a)satorlaser.com> wrote in message news:5acm27-40c.ln1(a)satorlaser.homedns.org... > Vladimir Grigoriev wrote: >> There is no a great sense to compare some size_t value with an object of >> type Person. This comparison has no sense. It is a general situation when >> converting some object of a class to an integral type has sense while >> converting an integral type to an object has no sense. For this purpose >> the explicit reywoard was introduced. The same is valid for operators. >> i.e. sometimes a op b has sense while b op a has no sense. > > I'd say whether switching operands still has sense depends on the > operator. > For an equality comparison, I expect it to be fully commutative. Similarly > for inequality, while e.g. greater behaves slightly different. > > Are you really implying that a==b makes sense when b==a doesn't? I already > voiced my opinion on comparison of a person with an ID: "I wouldn't > overload operator== for that." > > Uli > > -- > C++ FAQ: http://parashift.com/c++-faq-lite > > Sator Laser GmbH > Geschaftsfuhrer: Thorsten Focking, Amtsgericht Hamburg HR B62 932
From: sasha on 21 Jan 2010 14:28 Duane Hebert wrote: > > > What has the ctor for your class to do with the == operator for your class? > Maybe I'm missing someting... > > It may need to be constructed before compared - like when OP said during iteration of an array of IDs.
From: Duane Hebert on 21 Jan 2010 14:44
"sasha" <abc(a)fox.net> wrote in message news:O49e7$smKHA.6096(a)TK2MSFTNGP02.phx.gbl... > Duane Hebert wrote: >> > >> >> What has the ctor for your class to do with the == operator for your class? >> Maybe I'm missing someting... >> >> > > It may need to be constructed before compared - like when OP said during > iteration of an array of IDs. But if I have a ctor taking int, string and another taking string,int how does it relate to the == operator? This compares instances of an object that has already been constructed. I can have 27 ctors for a class and only one == operator. |