On Tue, Mar 12, 2019 at 03:26:05PM +0000, Victor Porton via Digitalmars-d-learn
wrote:
[...]
> On Tuesday, 12 March 2019 at 09:05:36 UTC, Nicholas Wilson wrote:
[...]
> > template FieldInfo(T) {
> > template FieldInfo(Nullable!(T) default_)
> > {
> > enum FieldInfo = 0;
> > }
> > }
> >
> > seems to work, but I can't seem to instantiate one of it.
>
> Why you use the same name "FieldInfo" for both the template and its
> subtemplate? Does it make some sense?
This is a D idiom called the "eponymous template". Whenever the
template contains a member of the same name as the template, it's an
eponymous template, and you can refer directly to the member by the
template name, rather than using templateName.memberName.
For example, a template function is usually written like this:
ReturnType myFunc(TemplateArgs...)(RuntimeArgs args...)
{
... // implementation here
}
This is actually shorthand for the eponymous template:
template myFunc(TemplateArgs...)
{
ReturnType myFunc(RuntimeArgs args...)
{
... // implementation here
}
}
Similarly, when you write:
enum isInputRange(T) = hasMember!(T, empty) && ...
that's actually shorthand for:
template isInputRange(T)
{
enum isInputRange = hasMember!(T, empty) && ...
}
The eponymonus template idiom allows you to use a single name to refer
to both the template and the member. Without this idiom, you'd have to
use the very verbose notation:
static if (isInputRange!T.isInputRange) ...
or
auto retval = myFunc!(A, B, C).myFunc(1, 2, 3);
T
--
It won't be covered in the book. The source code has to be useful for
something, after all. -- Larry Wall
