C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Elements of constant array as constant expressions.          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Andrzej 'Foxy' D. Guest Posted: Wed May 03, 2006 4:06 pm    Post subject: Elements of constant array as constant expressions. Could you explain me, why it's impossible to treat an element of constant array as a constant expression? Look at this code: const int array[] = { 1 }; template<int I> struct A {}; void f() { A<array[0]> a; // ! } It's obvious that a compiler can know, what's the value of array[0], isn't it? If an array is constant and the index is a constant expression, then the value of array[0] could be known at compile-time, treated as a constant expression and used e.g. as a template argument. What was the reason not to allow to use array's elements in this way? -- Pozdrawiam, Andrzej --- [ 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 David R Tribble Guest Posted: Fri May 12, 2006 7:23 pm    Post subject: Re: Elements of constant array as constant expressions. Andrzej 'Foxy' D. wrote: Quote: Could you explain me, why it's impossible to treat an element of constant array as a constant expression? Look at this code: const int array[] = { 1 }; template<int I struct A {}; void f() { A<array[0]> a; // ! } It's obvious that a compiler can know what's the value of array[0], isn't it? If an array is constant and the index is a constant expression, then the value of array[0] could be known at compile-time, treated as a constant expression and used e.g. as a template argument. What was the reason not to allow to use array's elements in this way? Because even though the compiler _could_ know, it is not _required_ to know by the language specification. Technically, the value of an array element (const or not) is not a compile-time constant. I know this doesn't sound right to some, but defining array elements as const does not automatically make them compile-time constants. The same is true of expressions like "abc"[0]. -drt --- [ 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 Victor Bazarov Guest Posted: Fri May 12, 2006 9:21 pm    Post subject: Re: Elements of constant array as constant expressions. David R Tribble wrote: Quote: Andrzej 'Foxy' D. wrote: Could you explain me, why it's impossible to treat an element of constant array as a constant expression? Look at this code: const int array[] = { 1 }; template<int I struct A {}; void f() { A<array[0]> a; // ! } It's obvious that a compiler can know what's the value of array[0], isn't it? If an array is constant and the index is a constant expression, then the value of array[0] could be known at compile-time, treated as a constant expression and used e.g. as a template argument. What was the reason not to allow to use array's elements in this way? Because even though the compiler _could_ know, it is not _required_ to know by the language specification. Technically, the value of an array element (const or not) is not a compile-time constant. I know this doesn't sound right to some, but defining array elements as const does not automatically make them compile-time constants. The same is true of expressions like "abc"[0]. I believe Andrzej's question wasn't "is it?" but rather "why isn't it?" We already know that the language specification doesn't require the elements of const arrays to be recognized as const expressions. The question is _why the hell not_. I'd like to know as well, BTW. V -- Please remove capital 'A's when replying by e-mail I do not respond to top-posted replies, please don't ask --- [ 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 Lucas Galfaso Guest Posted: Mon May 15, 2006 4:21 am    Post subject: Re: Elements of constant array as constant expressions. Hi all, This issue has come up in several different way over the years. You have to think that operator[] is a function, and, even when this function does not have side effects, the return value of a funcion is not a compile time constant. LG --- [ 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 Maciej Sobczak Guest Posted: Mon May 15, 2006 4:21 pm    Post subject: Re: Elements of constant array as constant expressions. Lucas Galfaso wrote: Quote: You have to think that operator[] is a function It's not how it's defined in 5.2.1/1: "The expression E1[E2] is identical (by definition) to *((E1)+(E2))." Quote: the return value of a funcion is not a compile time constant. But *((E1)+(E2)) could be. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ --- [ 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 ThosRTanner Guest Posted: Mon May 15, 2006 4:21 pm    Post subject: Re: Elements of constant array as constant expressions. Lucas Galfaso wrote: Quote: Hi all, This issue has come up in several different way over the years. You have to think that operator[] is a function, and, even when this function does not have side effects, the return value of a funcion is not a compile time constant. I don't see that you can make that deduction. For basic types (which an int[] array is), operators are not functions. Otherwise you wouldn't have logical shortcircuiting, and all the fun with order of evaluation issues. --- [ 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 Andrzej 'Foxy' D. Guest Posted: Mon May 15, 2006 9:21 pm    Post subject: Re: Elements of constant array as constant expressions. Victor Bazarov wrote: Quote: I believe Andrzej's question wasn't "is it?" but rather "why isn't it?" Exactly. And, what's funny, there haven't been any sufficient answer yet. -- Pozdrawiam, Andrzej --- [ 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 Marsh J. Ray Guest Posted: Thu Jun 01, 2006 7:20 pm    Post subject: Re: Elements of constant array as constant expressions. Andrzej 'Foxy' D. wrote: Quote: Could you explain me, why it's impossible to treat an element of constant array as a constant expression? It's obvious that a compiler can know, what's the value of array[0], isn't it? If an array is constant and the index is a constant expression, then the value of array[0] could be known at compile-time, treated as a constant expression and used e.g. as a template argument. What was the reason not to allow to use array's elements in this way? Array initializers, even for const arrays, don't seem to have nearly the constraints constraints as those for template type arguments. I just tested this on one compiler (MSVC80), but I don't think this is any vendor-specific extension. Consider what the compiler would have to do to instantiate your template given the following: int salesman_optimal_route_distance() { return 0; /* TODO: exercise for reader */ } int const array[] = { salesman_optimal_route_distance() }; template<int I> struct A { }; int main() { A<array[0]> a; } - Marsh --- [ 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 Victor Bazarov Guest Posted: Thu Jun 01, 2006 8:45 pm    Post subject: Re: Elements of constant array as constant expressions. "Marsh J. Ray" wrote: Quote: [..] Array initializers, even for const arrays, don't seem to have nearly the constraints constraints as those for template type arguments. I just tested this on one compiler (MSVC80), but I don't think this is any vendor-specific extension. But how is that related to the case when certain things _can_ be treated as compile-time constants? Quote: Consider what the compiler would have to do to instantiate your template given the following: int salesman_optimal_route_distance() { return 0; /* TODO: exercise for reader */ } int const array[] = { salesman_optimal_route_distance() }; Why would that demonstrate anything except that it's allowed? Try adding const int rtc = salesman_optimal_route_distance(); Is that a compile-time constant? No. Try const int ctc = 42; Is that a compile-time constant? Yes. What conclusions should we derive from that? Quote: template<int I struct A { }; int main() { A<array[0]> a; And change this to A<rtc> aa; What do you make of that? A<ctc> aaa; // should work just fine Quote: } V -- Please remove capital 'A's when replying by e-mail I do not respond to top-posted replies, please don't ask --- [ 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 Marsh J. Ray Guest Posted: Fri Jun 02, 2006 3:20 am    Post subject: Re: Elements of constant array as constant expressions. Victor Bazarov wrote: Quote: "Marsh J. Ray" wrote: [..] Array initializers, even for const arrays, don't seem to have nearly the constraints constraints as those for template type arguments. I just tested this on one compiler (MSVC80), but I don't think this is any vendor-specific extension. But how is that related to the case when certain things _can_ be treated as compile-time constants? Maybe you can prove that it can be treated as a compile-time constant, but can you construct a rule which allows what you are asking and show the standards committee that the implementers across an incredible range of platforms can teach their compilers to be able to prove that it can be treated as a compile-time constant? Also, don't forget that your array may have externally-visible linkage. <skip> Quote: What do you make of that? What I make of it is that the standards committee didn't want to require a compiler to have the entire content of (possibly large) arrays available at the time of template instantiation. - Marsh --- [ 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 Victor Bazarov Guest Posted: Fri Jun 02, 2006 5:20 am    Post subject: Re: Elements of constant array as constant expressions. "Marsh J. Ray" wrote: Quote: Victor Bazarov wrote: "Marsh J. Ray" wrote: [..] Array initializers, even for const arrays, don't seem to have nearly the constraints constraints as those for template type arguments. I just tested this on one compiler (MSVC80), but I don't think this is any vendor-specific extension. But how is that related to the case when certain things _can_ be treated as compile-time constants? Maybe you can prove that it can be treated as a compile-time constant, but can you construct a rule which allows what you are asking and show the standards committee that the implementers across an incredible range of platforms can teach their compilers to be able to prove that it can be treated as a compile-time constant? Just add 'elements of an array of const' to the list of what a constant expression "can involve" (in 5.19/1): "An integral constant-expression can involve only literals (2.13), enumerators, const variables, elements of arrays of const, or static data members of integral or enumeration types initialized with constant expressions (8.5), ..." Quote: Also, don't forget that your array may have externally-visible linkage. How is linkage playing into that now? Quote: skip What do you make of that? What I make of it is that the standards committee didn't want to require a compiler to have the entire content of (possibly large) arrays available at the time of template instantiation. This just doesn't make sense. What is so different between elements of an array of const int and a single const it? If the compiler can determine that a single int variable can be used in a compile-time constant expression, there is nothing seriously more difficult to do that for any element of an array. The Committee just didn't think of it, most likely. Let's hear from somebody on the Committee, shall we? V --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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