RE: c++/10273: g++ fails to compile sort() with a template operator that takes a pointer

[Wolfgang Bangerth <bangerth@ices.utexas.edu>]

The following reply was made to PR c++/10273; it has been noted by GNATS.

From: Wolfgang Bangerth <bangerth@ices.utexas.edu>
To: "Darren Reed (OSE)" <darrenr@optimation.com.au>
Cc: gcc-bugs@gcc.gnu.org, <gcc-gnats@gcc.gnu.org>
Subject: RE: c++/10273: g++ fails to compile sort() with a template operator
 that takes a pointer
Date: Mon, 31 Mar 2003 20:26:00 -0600 (CST)

 > >   x.cc: In function `void std::doit()':
 > >   x.cc:31: type/value mismatch at argument 1 in template parameter list for `
 > >    template<std::ClassName*<anonymous> > struct std::xLess'
 > >   x.cc:31:   expected a constant of type `std::ClassName*', got `std::ClassName*'
 > >
 > > I think the message says it quite clearly: first the type/value mismatch,
 > > then in addition that it expected a value and got a type. What more do you
 > > want?
 > 
 > Ok, so you know how to parse that message better than I do.  To me it says
 > "expected type 'A', got 'A'" (where A is "std::ClassName*") and hence it
 > does not make any sense to me. 
 
 No, read it, it says
     expected a constant [i.e. a value] of type 'X', got 'X' [which is a type]
 
 The situation is the same as if you had written
     template <int N> class X{};
 and then used it as
     X<int> x;
 That won't work.
 
 
 > I'm the programmer, so because I don't
 > intend for there to be a value there, I don't see a value there. 
 
 If you intend there to be a type, then you should declare the template as 
 taking a type, rather than a value.
 
 
 > The error
 > doesn't tell me that using * implies a value is being used in a template
 > where a type should be.
 
 What you wrote was 
     template <class ClassName *>  struct X {};
 which, since ClassName is the name of an existing class, is equivalent to
     template <ClassName *> struct X {};
 (that is just the same as declaring variables as  "struct X x;" or "X x;"
 is equivalent in C++.)
 
 
 > Just for the sake of comparison, I tried compiling the same piece of code
 > with Visual Studio .Net and SUNWspro CC.  Both of those compilers (rightly
 > or wrongly, I don't know) complain about the code when it comes to parsing
 > the template statement.  In my situation, this type of verbosity helps.
 > [...verbosity of other compilers deleted...]
 
 These compilers simply don't realize that
     template <class C *> struct X {};
 and
     template <C *>
 
 W.
 
 -------------------------------------------------------------------------
 Wolfgang Bangerth             email:            bangerth@ticam.utexas.edu
                               www: http://www.ticam.utexas.edu/~bangerth/