C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  semantics of == and copying          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Jens Theisen Guest Posted: Mon Sep 11, 2006 7:13 am    Post subject: semantics of == and copying Hello, I'm wondering what certain requirements in the standard precisely mean. The CopyConstructible (likewise Assignable) requirement says t and T(t) should be "equivalent". Is it specified anywhere what this actually means? Are they not equivalent if, say, foo(t) is different from foo(T(t)) for some foo? Clearly &t and &T(t) are different. The EqualityComparable requirement says that == should be an equivalence relation over its domain. However, I don't see a requirement for t == T(t). After all, a type does not have to be copyable in order to be EqualityComparable. Does the standard say something about the case when both requirements are met? Is t == T(t) required then? In particular, in the case of ForwardIterators, it is required that i == j implies ++i == ++j if both are dereferencable. This strongly appears to imply the latter, since it would otherwise be a rather meaningless requirement. Can anyone point me to the some information about this? Cheers, Jens --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Chris Jefferson Guest Posted: Mon Sep 11, 2006 3:41 pm    Post subject: Re: semantics of == and copying Jens Theisen wrote: Quote: Hello, I'm wondering what certain requirements in the standard precisely mean. The CopyConstructible (likewise Assignable) requirement says t and T(t) should be "equivalent". Is it specified anywhere what this actually means? Are they not equivalent if, say, foo(t) is different from foo(T(t)) for some foo? Clearly &t and &T(t) are different. The EqualityComparable requirement says that == should be an equivalence relation over its domain. However, I don't see a requirement for t == T(t). After all, a type does not have to be copyable in order to be EqualityComparable. Does the standard say something about the case when both requirements are met? Is t == T(t) required then? In particular, in the case of ForwardIterators, it is required that i == j implies ++i == ++j if both are dereferencable. This strongly appears to imply the latter, since it would otherwise be a rather meaningless requirement. Can anyone point me to the some information about this? Unfortunatly I believve that equivalent in this case is something which is very hard to exactly define. While there is no good definition about what it should mean, it is usually fairly obvious what it means in any context, meaning in general the copy "acts the same as the original". Like you say, this will not be true if you take it's address, but you can expect any iterator operations on both copies will produce the same results. I believe (someone else may correct me) there have been attempts to define exactly what equivalent means and there always ends up being a case where it doesn't act like people would reasonably expect. Chris --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Greg Herlihy Guest Posted: Mon Sep 11, 2006 3:42 pm    Post subject: Re: semantics of == and copying Jens Theisen wrote: Quote: Hello, I'm wondering what certain requirements in the standard precisely mean. The CopyConstructible (likewise Assignable) requirement says t and T(t) should be "equivalent". Is it specified anywhere what this actually means? Are they not equivalent if, say, foo(t) is different from foo(T(t)) for some foo? Clearly &t and &T(t) are different. No, the definition of CopyConstructible is not stating that t and T(t) are generally equivalent. Instead the equivalence applies to this expression: T(t) So for a CopyConstructible T class, the "t" in this expression is equivalent to "T(t)". In other words, replacing one with the other like so: T(T(t)) constructs a T object of the same value as the original. Quote: The EqualityComparable requirement says that == should be an equivalence relation over its domain. However, I don't see a requirement for t == T(t). After all, a type does not have to be copyable in order to be EqualityComparable. Does the standard say something about the case when both requirements are met? Is t == T(t) required then? No, there is no CopyConstructible requirement that the expression t == T(t) even has to be defined (often it will not be) - much less that such a test has to evaluate to true. Quote: In particular, in the case of ForwardIterators, it is required that i == j implies ++i == ++j if both are dereferencable. This strongly appears to imply the latter, since it would otherwise be a rather meaningless requirement. I don't see any connection between iterators and the CopyConstructible requirements. The iterator requirement is simply stating that the items in a container have a single, fixed order whenever a program iterates through the container. In other words, no two iterations through the same container will visit the elements in an order any different than the other. Greg --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top James Dennett Guest Posted: Mon Sep 11, 2006 10:18 pm    Post subject: Re: semantics of == and copying Greg Herlihy wrote: Quote: Jens Theisen wrote: Hello, I'm wondering what certain requirements in the standard precisely mean. The CopyConstructible (likewise Assignable) requirement says t and T(t) should be "equivalent". Is it specified anywhere what this actually means? Are they not equivalent if, say, foo(t) is different from foo(T(t)) for some foo? Clearly &t and &T(t) are different. No, the definition of CopyConstructible is not stating that t and T(t) are generally equivalent. That's what it's intended to do though; it's saying that the copy constructor really does make copies. Quote: Instead the equivalence applies to this expression: T(t) So for a CopyConstructible T class, the "t" in this expression is equivalent to "T(t)". In other words, replacing one with the other like so: T(T(t)) constructs a T object of the same value as the original. A copy constructor that returned a constant value, ignoring its argument, would satisfy that condition, as would any other idempotent operation such as a projection. That's not what is wanted. Quote: The EqualityComparable requirement says that == should be an equivalence relation over its domain. However, I don't see a requirement for t == T(t). After all, a type does not have to be copyable in order to be EqualityComparable. Does the standard say something about the case when both requirements are met? Is t == T(t) required then? No, there is no CopyConstructible requirement that the expression t == T(t) even has to be defined (often it will not be) - much less that such a test has to evaluate to true. There's a clear implication, however, that if == is defined with its normal meaning then it should evaluate to true. -- James --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Jens Theisen Guest Posted: Tue Sep 12, 2006 4:40 am    Post subject: Re: semantics of == and copying "Chris Jefferson" <4zumanga (AT) gmail (DOT) com> writes: Quote: Unfortunatly I believve that equivalent in this case is something which is very hard to exactly define. While there is no good definition about what it should mean, it is usually fairly obvious what it means in any context, meaning in general the copy "acts the same as the original". Like you say, this will not be true if you take it's address, but you can expect any iterator operations on both copies will produce the same results. What led me to this question were considerations about forward iterators, which require s == t -> ++s == ++t. However, if not even t == T(t) is required than this other requirement is worth little. A forward iterator's operator == could be implemented by yielding true only comparing two past-the-end values. All other comparisons can evaluate to false without formally braking the requirements. That's probably not the intention of the standard. Is this a defect? Jens --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top kanze Guest Posted: Tue Sep 12, 2006 6:35 pm    Post subject: Re: semantics of == and copying Jens Theisen wrote: Quote: "Chris Jefferson" <4zumanga (AT) gmail (DOT) com> writes: Unfortunatly I believve that equivalent in this case is something which is very hard to exactly define. While there is no good definition about what it should mean, it is usually fairly obvious what it means in any context, meaning in general the copy "acts the same as the original". Like you say, this will not be true if you take it's address, but you can expect any iterator operations on both copies will produce the same results. What led me to this question were considerations about forward iterators, which require s == t -> ++s == ++t. However, if not even t == T(t) is required than this other requirement is worth little. A forward iterator's operator == could be implemented by yielding true only comparing two past-the-end values. All other comparisons can evaluate to false without formally braking the requirements. That's probably not the intention of the standard. Is this a defect? Good question. In practice, I've had no trouble with iterators which used a.isAtEnd() == b.isAtEnd() as the implementation of ==: two iterators are equal if both are at the end of the sequence, or neither is. I don't think that the intent was that this be guaranteed, but is seems to work in practice (although it doesn't meet the formal requirements you mention). I've been unable to find anything more precise either. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Greg Herlihy Guest Posted: Wed Sep 13, 2006 4:03 pm    Post subject: Re: semantics of == and copying Jens Theisen wrote: Quote: "Chris Jefferson" <4zumanga (AT) gmail (DOT) com> writes: Unfortunatly I believve that equivalent in this case is something which is very hard to exactly define. While there is no good definition about what it should mean, it is usually fairly obvious what it means in any context, meaning in general the copy "acts the same as the original". Like you say, this will not be true if you take it's address, but you can expect any iterator operations on both copies will produce the same results. What led me to this question were considerations about forward iterators, which require s == t -> ++s == ++t. However, if not even t == T(t) is required than this other requirement is worth little. X(a) == a is a necessary precondition for forward iterators (it's listed third in the table of forward iterator requirements). Greg --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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