cppannotations/yo/advancedtemplates/iterating.yo

    As there are no variables in template meta programming, there is no
template equivalent to a tt(for) or tt(while) statement. However, iterations
can always be rewritten as recursions.  Recursions em(are) supported
by templates and so iterations can always be implemented as (tail) recursions.
        hi(templates: iteration by recursion)

    To implement iterations by (tail) recursion do as follows:
    itemization(
    it() define a specialization implementing the end-condition;
    it() define all other steps using recursion.
    it() store intermediate values as tt(enum) values.
    )
    The compiler selects a more specialized template implementation over a
more generic one. By the time the compiler reaches the end-condition the
recursion stops since the specialization does not use recursion.

    Most readers will be familiar with the recursive implementation of the
mathematical `em(factorial)' operator, commonly represented by the exclamation
mark (tt(!)). Factorial tt(n) (so: tt(n!)) returns the successive products
tt(n * (n - 1) * (n - 2) * ... * 1), representing the number of ways tt(n)
objects can be permuted. Interestingly, the factorial operator is itself
usually defined by a em(recursive) definition:
        verb(
    n! = (n == 0) ?
            1
        :
            n * (n - 1)!
        )
    To compute tt(n!) from a template, a template tt(Factorial) can be defined
using an tt(int n) template non-type parameter. A specialization is defined
for the case tt(n == 0). The generic implementation uses recursion according
to the factorial definition. Furthermore, the tt(Factorial) template defines
an tt(enum) value `tt(value)' containing its factorial value. Here is the
generic definition:
        verb(
    template <int n>
    struct Factorial
    {
        enum { value = n * Factorial<n - 1>::value };
    };
        )
    Note how the expression assigning a value to `tt(value)' uses constant
values that can be determined by the compiler. The value n is provided, and
tt(Factorial<n - 1>) is computed using em(template meta
programming). tt(Factorial<n-1>) in turn results in value that can be
determined by the compiler (em(viz.)
tt(Factorial<n-1>::value)). tt(Factorial<n-1>::value) represents the tt(value)
defined by the em(type) tt(Factorial<n - 1>). It is em(not) the value returned
by an em(object) of that type. There are no objects here but merely values
defined by types.

The recursion ends in  a specialization. The compiler will select the
specialization (provided for the terminating value 0) instead of  the generic
implementation whenever possible. Here is the specialization's implementation:
        verb(
    template <>
    struct Factorial<0>
    {
        enum { value = 1 };
    };
        )
    The tt(Factorial) template can be used to determine, compile time, the
number of permutations of a fixed number of objects. E.g.,
        verb(
    int main()
    {
        cout << "The number of permutations of 5 objects = " <<
                Factorial<5>::value << "\n";
    }
        )
    Once again, tt(Factorial<5>::value) is em(not) evaluated at run-time, but
at compile-time. The run-time equivalent of the above tt(cout) statement is,
therefore:
        verb(
    int main()
    {
        cout << "The number of permutations of 5 objects = " <<
                120 << "\n";
    }
        )