Applying storage to type tuples

[Burton Radons]

Daniel Keep Wrote:

> 
> 
> Burton Radons wrote:
> > I'm writing a couple of modules for dealing with database values (well really it's just storage of aggregates) in a native fashion from within D, using D 2.023. I have a tuple called FieldTypes which contains the D-side types. I'm trying to use it to implement opApply:
> > 
> >     int opApply (int delegate (FieldTypes) func)
> > 
> > Unfortunately, this fails because the compiler only accepts the field types if they're references. But if I do this:
> > 
> >     int opApply (int delegate (ref FieldTypes) each)
> > 
> > It seems that the delegate takes a reference to one type, which is a value tuple! For example it claims that, simplified, "function Table.opApply (int delegate (ref (string, string)) each) does not match parameter types (int delegate (ref string, ref string))".
> > 
> > I've already implemented another opApply that takes a Line containing all the fields, but I'd like to have the expanded form as well if possible. Is it?
> 
> Yeah, I've run into this before.  I solved it at the time using CTFE and
> string mixins:
> 
> alias mixin(refDelegateFromTuple!(FieldTypes)) FieldTypesDg;

I couldn't quite figure out what you meant by this as DelegateFromTuple's not part of Phobos from what I can tell, but I did get this going:

	pure string BuildArguments (string prefix, T...) ()
	{
		string text;
		
		foreach (type; T)
		{
			if (text.length) text ~= ", ";
			text ~= prefix ~ " ";
			text ~= type.stringof;
		}
		
		return text;
	}
	
	alias TypeTuple! (int, float) FieldTypes;
	mixin ("alias int delegate (" ~ BuildArguments! ("ref", FieldTypes) () ~ ") FieldTypesDeg;");

Seems to work fine, I just don't know whether "stringof" is going to be well-behaved here. Plus it's a little extra work for the compiler.