Question

I have been reading about the template system in the D language and came upon a unusual construct, static if.

From what I managed to grasp it is evaluated at compile time, but from what I have searched, the example shown here did not quite enlighten me.

template Factorial(ulong n)
{
    static if(n < 2)
        const Factorial = 1;
    else
        const Factorial = n * Factorial!(n - 1);
}

What does static if do, and when should I use it?

Answer 1

the D static if is the base for “conditional compilation”, and plays an important role wherever a compile time decision about a variant of a code must be taken.

Since D doesn’t have a preprocessor, things like

#ifdef xxx
compile_this_piece_of_code
#endif

can become

static if(xxx)
{
     compile_this_pece_of_code
}

similarly, metaprogramming can happen also via static if:

template<int x>
struct traits
{ some definition calculated from x };

template<>
struct traits<0>
{ same definitions for the 0 particular case }

can be

template(int x)
{
    static if(x==0)
    { some definitions }
    else
    { some other same definitions }
    even more definition common in the two cases
}