On Saturday, 24 October 2015 at 11:04:14 UTC, Edwin van Leeuwen
wrote:
I am trying to write a function to merge two named structs, but
am completely stuck on how to do that and was wondering if
anyone good provide any help. I know I can access the different
names with tup.fieldNames, but basically can't work out how to
use that to build the new return type. Below is an outline of
what I am trying to do (with unittest). Any pointers would be
greatly appreciated.
I tried the following, but get a compile error:
source/ggplotd/aes.d(633): Error: variable tup cannot be read at
compile time
source/ggplotd/aes.d(633): Error: argument to mixin must be a
string, not (__error)
source/ggplotd/aes.d(646): Error: template instance
ggplotd.aes.merge!(Tuple!(string[], "x", string[], "y", string[],
"label"), Tuple!(double[], "x", double[], "y")) error
instantiating
import std.typecons : Tuple;
template merge(T, U)
{
auto merge( T base, U other )
{
string typing = "Tuple!(";
string variables = "(";
foreach( i, t; other.fieldNames )
{
typing ~= other.Types[i].stringof ~ ",\"" ~ t ~ "\",";
variables ~= "other." ~ t ~ ",";
}
foreach( i, t; base.fieldNames )
{
bool contains = false;
foreach( _, t2; other.fieldNames )
{
if (t==t2)
contains = true;
}
if (!contains)
{
typing ~= base.Types[i].stringof ~ ",\"" ~ t ~
"\",";
variables ~= "base." ~ t ~ ",";
}
}
string tup = typing[0..$-1] ~ ")" ~ variables[0..$-1] ~
");";
// Do some clever CTFE
return mixin(tup);
}
}
///
unittest
{
auto xs = ["a","b"];
auto ys = ["c","d"];
auto labels = ["e","f"];
auto aes = Tuple!(string[], "x", string[], "y", string[],
"label")(
xs, ys, labels );
auto nlAes = merge( aes, Tuple!(double[], "x",
double[], "y" )(
[0,1], [3,4] ) );
assertEqual( nlAes.x[0], 0 );
assertEqual( nlAes.label.front, "e" );
}
I guess fieldNames does not exist at compile time? Can I get the
fieldNames etc at compile time?
Cheers, Edwin
