Const and usage during compile time
in [C++]
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From: krishna on 11 Nov 2009 15:37 const int i[] = {10, 12, 18, 20} //The below isn't legal, the text says because the array above being const, it can't be used at compile time, and the error is "error C2057: expected constant expression". It seems (I tried it) it can't be done whether the array above is const or not, the (here float)array size value must be a constant rather than say an expression. Is this true? float f[i[1]]; what was the author (Bruce Eckel) trying to say (page 338 Vol 1 second Edition, Thinking in c++) by giving const array example in this case? -Krishna. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: John H. on 12 Nov 2009 09:11 krishna wrote: > const int i[] = {10, 12, 18, 20} > //The below isn't legal, the text says because the array above being > const, it can't be used at compile time, and the error is "error > C2057: expected constant expression". It seems (I tried it) it can't > be done whether the array above is const or not, the (here float)array > size value must be a constant rather than say an expression. Is this > true? According to C++, an array size must be a "integral constant expression" which has a particular meaning defined by the standard (5.19). Many of things you might think of as integral constant expression are (e.g. const int x = 3;). Accessing an element from an array does not qualify (even though it is an array of const ints). It seems to me like the compiler should be able to determine the value at compile time and thus use it as an array size. I am not sure of the reasoning of excluding it from things allowed as integral constant expressions. Another related concept is variable length arrays. This is a feature of C99 that many C++ compilers support. With this feature, the size of the array needn't be a constant expression, and the code you suggested would work. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: liam on 12 Nov 2009 09:15 > array size value must be a constant rather than say an expression. Is this > true? Yes it must be a compile time constant as VLA's are not supported by C+ +. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Alp Mestan on 13 Nov 2009 03:41
On Nov 13, 3:11 am, "John H." <oldman_fromt...(a)yahoo.com> wrote: > According to C++, an array size must be a "integral constant > expression" which has a particular meaning defined by the standard > (5.19). Many of things you might think of as integral constant > expression are (e.g. const int x = 3;). Accessing an element from an > array does not qualify (even though it is an array of const ints). Absolutely. Only a compile-time array, like boost::mpl::vector_c, can let you do that, but yeah it's not the same kind of container anymore. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |