Re: How do you test whether a variable is static or not?

[Basile B. via Digitalmars-d-learn]

On Saturday, 30 July 2016 at 12:47:10 UTC, Jonathan M Davis wrote:
I'm writing some serialization code where I need to skip static 
variables. So, I have a symbol from a struct, and I'd like to 
test whether it's static or not. Ideally, I'd be able to do 
something like

is(field == static)

but of course that doesn't work. There is 
__traits(isStaticFunction, ...),
but there is no __traits(isStaticVariable, ...) or 
__traits(isStatic, ...),
and I don't see anything in std.traits which would help. The 
closest that
I've been able to come up with is

template isStaticVar(T, alias field)
{
    enum isStaticVar =
        is(typeof({mixin("auto i = T." ~ field.stringof ~ 
";");}));
}

For a more general solution, it would of course need to verify 
that the field wasn't a function as well, but in my case, that 
was already checked prior to instantiating isStaticVar, so I 
can skip it here. And what I have mostly works. However, it 
would fail miserably if the field were a static member variable 
that was non-copyable. If it were, then that code would would 
declare it to be non-static. Now, non-copyable structs are rare 
enough that I can probably get away with letting that bug 
stand, but I'd prefer to correctly detect whether a variable is 
static or not. Does anyone have a better idea of how to do this?

- Jonathan M Davis

You won't be able to assign it if it's not static. Given this


    isStaticVar(Stuff) = __traits(compiles, Stuff = 
typeof(Stuff).init))

Unfortuantely it's hard to make a stand alone template for this. 
For example I was not able to verify the non-static case


struct Foo
{
    static int a;
    int b;
}

template isStaticVar(alias stuff)
{
    bool check()
    {
        bool result;
        static if (!is(typeof(__traits(parent, stuff))))
            result = true;
        else static if (__traits(compiles, stuff = 
typeof(stuff).init))
            result = true;
        return result;
    }
    enum isStaticVar = check();
}

unittest
{
    static assert(isStaticVar!(Foo.a));
    //static assert(isStaticVar!(Foo.b));
}

I'm curious to see the other solutions.