On Thursday, 28 April 2016 at 10:21:34 UTC, Andrew Benton wrote:
I'm running into a set of problems where both inheritance and
mixin-added functionality really lend a hand to my project.
Unfortunately, they don't necessarily work well together, as
certain mixins need to be restated for each inheritor.
As a toy example, if I wanted to mixin some functionality to
write names:
mixin template WriteName()
{
string writeName()
{
import std.uuid : randomUUID;
static if(__traits(hasMember, this, "name"))
return __traits(getMember, this, "name);
else
return randomUUID.toString;
}
}
class Base
{
mixin WriteName;
}
class Inheritor : Base
{
version(both) mixin WriteName;
string name = "test";
}
In the above example, unless I used version "both", I would see
only random UUIDs from instances of Base and Inheritor, however
the functionality that I really want is to get random UUIDs
only from Base, and get "test" from Inheritor. So far as I
know, the easiest way to fix this is to add that mixin to each
descendant of Base.
I recognize that, at least for this toy example, an easier fix
is to use a templated function, however I'm more interested in
libraries like jsonizer and witchcraft.
So to the point: Is there an easier way to do this that I'm
missing? Is there a language-design reason that mixed in
templates can't inherit? It seems like an intuitive use of
mixin.
This is a common error with mixin template. The static things
gets only evaluated once. If you want to be evaluated in each
class type you need a true template that get an instance in each
class. However the method then **wont be virtual** !!
----
mixin template WriteName()
{
string writeName(this T)()
{
import std.uuid : randomUUID;
static if (__traits(hasMember, T, "name"))
return __traits(getMember, cast(T)this, "name");
else
return randomUUID.toString;
}
}
class Base
{
mixin WriteName;
}
class Inheritor : Base
{
string name = "test";
}
void main()
{
import std.stdio;
writeln((new Base).writeName!Base); // an UUID
writeln((new Inheritor).writeName!Inheritor); // test
}
----
see template this parameter:
https://dlang.org/spec/template.html#TemplateThisParameter
