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

[Bosak]

On Friday, 2 August 2013 at 11:52:32 UTC, monarch_dodra wrote:
> 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).

Oh I like your solution. It is just what I needed, thanks! Also I 
didn't knew you could make enums that contain string values. So 
in D enums are like constants in C#, right? I see that enums are 
used a lot with templates. And why is the name of the enum the 
same with the name of the template? Does it allways have to be 
like that?