Is there a cleaner way of doing this?

[Dgame via Digitalmars-d]

On Monday, 7 August 2017 at 08:01:26 UTC, Shachar Shemesh wrote:
> It is often desired to have a struct with an extra parameter. 
> The common way to do this is like so:
>
> struct S(T) {
> 	T param;
>
> 	void initialize(T param) {
> 		this.param = param;
> 		// Other stuff
> 	}
> }
>
> The problem is what happens when the param is optional. The 
> common way to do this is to set T to void. This results in the 
> following code:
>
> struct S(T) {
> 	enum HasParam = !is(T == void);
> 	static if( HasParam ) {
> 		T param;
> 	}
>
> 	static if( HasParam ) {
> 		void initialize(T param) {
> 			this.param = param;
> 			// Other stuff
> 		}
> 	} else {
> 		void initialize() {
> 			// Same other stuff as above!
> 		}
> 	}
> }
>
> This is both tedious and error prone. Is there a cleaner way of 
> doing this?
>
> Just as an unrealistic fantasy, if the following code was 
> legal, the problem would be resolved on its own:
> void func(void p) {
> 	void param;
>
> 	param = p;
>
> 	return param;
> }
>
>
> Of course, that code has its own set of problems, and I'm not 
> really suggesting that change.
>
> Shachar

Why don't you use a factory method?

struct S(T)
{
     T p;

     static make(T p)
     {
         S s;
         s.p = p;

         return s;
     }

     static make()
     {
         return S();
     }
}

void main()
{
     auto s1 = S!int.make;
     auto s2 = S!string.make;
}