TITLE: restricting constant template arguments

(Newsgroups: comp.lang.c++,gnu.g++.help, 6 Oct 99)

[ This shows a technique that relies on compilers enforcing
  that multiple definitions for typedef must be the same. -adc ]


FREEMAN: Andy Freeman <anamax@earthlink.net>

> I'd like to define a template for functions which must have constant
> arguments.  ("Must" comes from another use of the code that uses
> these functions.)


KUEHL: Dietmar Kuehl <dietmar.kuehl@claas-solutions.de>

That one is easy: Pass the arguments as template arguments:

  template <bool> struct tassert { typedef int assertion; };
  template <> struct tassert<false> {};
  struct foo {
    template <int H, int L, int S>
    int f()
    {
      typedef typename tassert<(H > S && H >= L && L > 0)>::assertion
              failed;
      return H + L + S;
    }
  };

  int main() {
    foo f;
    return f.f<5, 2, 3>();
  }

This also turns the assertions into compile time assertions.

[snip]