On Tuesday, 12 March 2019 at 16:20:11 UTC, H. S. Teoh wrote:
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);
I know what is eponymous template. But how it behaves when the
eponymous member inside itself is also a template? How to
instantiate it? (provide please an example how to instantiate)
