alias to a property as an argument to a mixin template

[Philippe Sigaud]

monarch_dodra already answered, but since, I typed this, I may as well
post it :)


On Sun, Sep 23, 2012 at 8:49 PM, comco <void.unsigned at gmail.com> wrote:
> For this program I'm getting an "Error: need 'this' to access member x" at
> line (*). Does that mean that we cannot alias a property as an argument of a
> template mixin?

By using s.x, you're not referencing the property, but directly the
value s.x, which is known only at runtime: it cannot be a template
argument.

If x where a static member, you could probably use it, hence the error
message (need 'this', ...).

> So, using string mixins works, but explicit alias to the property name seems
> not to. Why is that?

a.stringof can be obtained for any symbol a, so s.x (or with(s) ... x)
just gives "s.x", which can be mixed in. It's transformed into a
string, transformation for which there is no need for 'this'.

>  and is there any other way of achieving the result
> witout using template mixins

Sorry but... what result? Referencing a member inside a template?
Remember templates can be in another module, written years ago. If you
really want a template to act on a local value, either use a mixin
template, as you did, or reference the member by its name as a string:

import std.stdio;

mixin template T(string member)
{
    void f()
    {
        mixin("writeln(" ~ member ~ ");");
    }
}

struct S
{
    int x;
}

void main() {
    auto s = S(4);

    mixin T!("s.x");
    f();

} // prints 4

I see monarch proposed exactly the same way to do it...