How to create mixin template and pass name of a variable?

[monarch_dodra]

On Friday, 2 August 2013 at 11:37:27 UTC, Bosak wrote:
> I want to create a mixin template such that:
>
> mixin template ArgNull(alias arg, string name)
> {
>     if(arg is null)
>         throw new Exception(name~" cannot be null.");
> }
>
> But is there a way to do that with only one template argument. 
> And then use it like:
>
> string text = null;
> mixin ArgNull!(text);
>
> And the above mixin to make:
>
> if(text is null)
>     throw new Exception("text cannot be null.");

A mixin template can't inject arbitrary code (AFAIK), but only 
members/functions/structs etc.

A simple string mixin would solve what you need better anyways:

import std.stdio, std.typetuple, std.string;

//----
template ArgNull(alias name)
{
     enum ArgNull = format(q{
        if(%1$s is null)
            throw new Exception("%1$s cannot be null.");
     }, name.stringof);
}

void main()
{
     string s1 = "hello";
     string s2;

     mixin(ArgNull!s1);
     mixin(ArgNull!s2);
}
//----

FYI, "q{}" is called a "token string", and is formidably useful 
to write a string that contains code. Also, I'm exploiting the 
alias arg to extract the name out of the variable. This makes it 
impossible to accidentally call the mixin with an invalid string 
(the error will be *at* the mixin call, not *in* the mixin call).