On Fri, 13 Jan 2012 22:21:52 +0100, Philippe Sigaud
<[email protected]> wrote:
There is an 'Examples' section where I show what can be done with
templates and there I
'borrowed' some code posted here, with attribution. I already exchanged
with Andrej
Mitrovic (thanks!), but also took some code from Timon Gehr, Simen
Kjaeraas, Trass3r
and Jacob Carlborg. Guys, if any of you have a problem with that, tell
me so and I'll
take down the code of course. But if any of you could give me some
explanation (a small
paragraph or two?) about what your code does, I'll be forever grateful :)
The extended enum example does not compile, because you've removed the
unittest{} block around the the tests. I'd say the code in your document
should compile straight out of the box, to be newb-friendly.
As for an explanation:
string EnumDefAsString(T)() if (is(T == enum)) {
string result = "";
foreach (e; __traits(allMembers, T))
result ~= e ~ " = T." ~ e ~ ",";
return result;
}
This piece of code iterates over all members of the passed enum T,
generating a string containing all members and their values. For
this enum:
enum bar {
a, b, c
}
the generated string looks like this (if you want to check this,
feel free to call EnumDefAsString at run-time and print its result):
"a = bar.a,b = bar.b,c = bar.c"
As we can see, this is a valid body for an enum. That means we can use
mixin() to generate this exact same enum. But wait - there's more:
template ExtendEnum(T, string s)
if (is(T == enum) &&
is(typeof({mixin("enum a{"~s~"}");})))
{
mixin(
"enum ExtendEnum {"
~ EnumDefAsString!T()
~ s
~ "}");
}
This code concatenates the string generated from the previous function
with that passed to the function as parameter s. So with bar previously
defined, and this instantiation:
ExtendEnum!(bar, "d=25")
the body of the function will look like this (after string expansion):
mixin(
"enum ExtendEnum {"
~ "a = bar.a,b = bar.b,c = bar.c"
~ "d=25"
~ "}");
concatenating those strings, we see that we have a valid enum definition:
enum ExtendEnum {a = bar.a,b = bar.b,c = bar.c,d=25}
The mixin then pastes it in, and it is compiled as regular D code.
TLDR:
This code generates an enum definition as a string, by taking all the
members of the old enum, and adding those passed in string parameter s,
and mixing it in.
