On Sun, Apr 19, 2009 at 7:19 PM, James Dennett <james.denn...@gmail.com> wrote: > On Sun, Apr 19, 2009 at 4:15 PM, Paolo Piacentini > <paolopi...@hotmail.com> wrote: >> I don't think this is a bug but certainly it is a problem. >> >> Would you please consider it and let me know? I hope so. Thanks. >> >> The following simple volcalc.cpp code compiles with no errors (and >> works) in Windows Visual C++. >> It simply sizes the "alldata" array later in the code. > > If Visual C++ does not diagnose the error in the code in its best > standards-conforming mode, that is a bug in Visual C++. Allowing it > as an extension is an entirely reasonable thing to do though. > >> With g++ v.4.3.2 instead I get the error reported hereby. >> For some reason it does not like the fact that struct is declared >> local. > > That is what C++ requires; template parameters must have external > linkage, and in C++98 local types do not have external linkage. > >> If you declare struct as global it will be working but I cannot >> change the code so drastically. >> >> I would thankfully appreciate any help (including tough critics to the code). > > The "drastic" change would be needed to make your code valid C++, and > if you do that then g++ will compile it. > > There has been discussion of changing this rule for the next C++ > standard, see e.g., > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2402.pdf > though I don't see signs of it having been merged into the current > committee draft. N2402 does mention the change in MSVC++ as being a > relatively recent extension. A quick search hasn't turned up the > current status of N2402, though there was some discussion of weakening > it by removing support for using unnamed types as template parameters, > and it seemed to have reasonably strong support in that form.
Asking around a little more, and I've been pointed to http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm which has been incorporated into the current committee draft for C++0x, and allows use of local types as template parameters. It looks like your code will become legal in the next C++ standard, and g++ might well start supporting this before then (if someone is inspired to implement it, that is). -- James
