Is there a cleaner way of doing this?

[Diego 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

You can use type default initialization property:

struct S(T) {
	T param;

	void initialize(T param = T.init) {
		this.param = param;
		// Other stuff
	}
}

S!int s;
s.initialize(42);  // works
s.initialize();  // also works; s.param == int.init == 0