Re: c++/7181: foo<n>::bar = foo<n-1>::bar + foo<n-2>::bar evaluates to zero at compile time

[Nickolai Dobrynin <dobrynin@miller.cs.uwm.edu>]

The following reply was made to PR c++/7181; it has been noted by GNATS.

From: Nickolai Dobrynin <dobrynin@miller.cs.uwm.edu>
To: Paolo Carlini <pcarlini@unitus.it>
Cc: <gcc-gnats@gcc.gnu.org>, <gcc-prs@gcc.gnu.org>, <dobrynin@bigfoot.com>,
   <gcc-bugs@gcc.gnu.org>, Mark Mitchell <mark@codesourcery.com>,
   Nathan Sidwell <nathan@codesourcery.com>
Subject: Re: c++/7181: foo<n>::bar = foo<n-1>::bar + foo<n-2>::bar evaluates
 to zero at compile time
Date: Tue, 2 Jul 2002 15:52:27 -0500 (CDT)

 Hello!
 
 Actually, the original example from the "Generative Programming" book
 looked like this:
 
 #include <iostream>
 using namespace std;
 
 template<int n>
 struct Fib
 { enum { RET = Fib<n-1>::RET + Fib<n-2>::RET };
 };
 
 template<>
 struct Fib<0>
 { enum { RET = 0 };
 };
 
 template<>
 struct Fib<1>
 { enum { RET = 1 };
 };
 
 void main()
 { cout << Fib<8>::RET << endl;
 }
 
 -- which, evidently, is not identical to what I was submitting in my
 original message. After eliminating the "enum hack" (an optional step),
 i.e. replacing enum with const static int,
 
 #include <iostream>
 using namespace std;
 
 template<int n>
 struct Fib { const static int RET = Fib<n-1>::RET + Fib<n-2>::RET; };
 
 struct Fib<0> { const static int RET = 0; };
 struct Fib<1> { const static int RET = 1; };
 
 int main()
 {
   cout << Fib<40>::RET << endl;
 
 
   return 0;
 }
 
 this will indeed produce a valid output.
 
 I just think of it as being totally counter intuitive that the same nifty
 feature is not valid (is NO LONGER valid, would be a better way to say it)
 on non-constant initializers.
 
 So, would indeed this be possible to restore the behavior of 2.95.x or it
 would come in conflict with something else? Any opinions on that?
 
 
 Regards,
 
 Nickolai
 
 
 PS  Is it a normal behavior that the option -ftemplate-depth-... is no
 longer needed in order for the example above to compile properly?
 
 
 On Tue, 2 Jul 2002, Paolo Carlini wrote:
 
 > Hi,
 >
 > from a very practical point of view, would be difficult to restore the
 > behaviour of 2.95.x? Note that Intel and Comeau adopts that "particular"
 > initialization order and the current "equivalent" one ;-) breaks a whole
 > body of literature on template metaprogramming...
 >
 > Ciao, Paolo.
 >
 > http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7181
 >
 >
 >