On Wed, 1 Sep 2004 14:26:30 -0400, Arkadiy Vertleyb <vertleyb@hotmail.com> wrote:
"Peder Holt" <peder.holt@gmail.com> wrote
As to the implementation of my code, I got the inspiration to my implementation from Daniel Wallins post some time back on compile time constants: http://lists.boost.org/MailArchives/boost/msg69842.php
I compiled this code in VC7.1, and it did compile although with warnings.
But then I modified main to look like this:
int main() { SET(X, 10); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; SET(X, 15); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; SET(X, 20); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; SET(X, 15); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; }
the output was somewhat different than one would expect:
1 10 2 15 3 20 3 -- wrong 20 -- wrong
How do you handle this in your typeof implementation? Do you reset it somehow between typeofs? Or do you start from the next available number?
That is because the class set_variable<X,15> has already been instantiated. When you instantiate it again, you get the old implementation of set_variable. To overcome this problem, you need to add an additional template parameter to the set_variable class: template<int N> struct counter : counter<N - 1> {}; template<> struct counter<0> {}; template<int N> struct size_type { typedef char(&type)[N + 1]; }; size_type<0>::type check(...); template<class T, int Current, int N> struct set_variable { typedef counter<10> start_type; enum { current = Current, next = current + 1 }; friend typename size_type<next>::type check(T*, counter<next>*) {} friend typename size_type<N>::type value(T*, counter<current>*) {} }; #define CURRENT(T) (sizeof(check((T*)0, (counter<10>*)0)) - 1) #define VALUE(T) (sizeof(value((T*)0, (counter<10>*)0)) - 1) #define SET(T, N) set_variable<T,CURRENT(T), N>() #include <iostream> struct X {}; int main() { SET(X, 10); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; SET(X, 15); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; SET(X, 20); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; SET(X, 15); std::cout << CURRENT(X) << "\n"; std::cout << VALUE(X) << "\n"; } voila. -- Peder Holt
Regards, Arkadiy
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