Query regarding the use/limitiations of traits in metaprogramming

From: hicks (hicks_at_[hidden])
Date: 2002-05-05 23:13:37

Query regarding the use/limitiations of traits in metaprogramming

I am trying to use traits pattern to assist in metaprogramming.
I have the following problem.

A traits class "trait" defers a decision to another traits class 
"trait1" depending
on the result of a third traits class "traita".  
However, the compiler does not cooperate.
The code below excutes but gives unexpected output (also shown).

[Note:: The code has been reduced to the minimal to demonstrate the problem.
I have a real task at hand not shown here.]

template <class T>
struct traita { BOOST_STATIC_CONSTANT(int, value=0); };
template <>
struct traita<int> { BOOST_STATIC_CONSTANT(int, value=1); };

template <class T, int>
struct trait1 {};

template <class T>
struct trait
{
    BOOST_STATIC_CONSTANT(int, value=traita<T>::value);
    typedef trait1<T, value> t1;
};

int main(int argc, char* argv[])
{
    std::cout
    << typeid(trait<int>::t1).name()
    << std::endl
    << trait<int>::value
    << std::endl
    ;
    return 0;
}

output::

trait1<int,0>
1

It would be nice if the first line of output were "trait1<int, 1>", but 
it isn't.

Any assistance would be greatly appreciated.

Craig Hicks