On Monday, 10 April 2017 at 21:04:10 UTC, Johan Fjeldtvedt wrote:
I have a couple of questions related to the following code:
https://gist.github.com/Jaffe-/b027287a884fc2e173a65601ec242676
1) This is a very simplified example, but what I'm trying to do
here is to call `foo` on each object in `Container.ss` contains
when `foo` is called, and likewise for `bar`. To do this
without having to repeat the foreach loop in every member
function, I made the `call_for_each` template. However, I found
no other way to pull this off (without using string mixins)
than to also add the `call` template in the S struct. At first
I thought it would be possible to write `s.func(args)` in
`call_for_each`, but that will try to look up func as a member
in the struct.
Is there a more common / idiomatic way of doing this? In C++
this would be solved by using a member function pointer as a
template argument, but as far as I understand D uses delegates
(which are already bound to an instance) instead?
One way:
struct S {
int x[];
void foo(int x) {
this.x ~= x;
}
void bar() {
writeln("Contains: ", x);
}
auto call(alias func, T...)(T args) {
return func(args);
}
auto reduce(alias func)() {
return x.reduce!(func);
}
}
class Container {
S[10] ss;
void dispatch(string func, T...)(T args) {
foreach(ref s; ss) {
__traits(getMember, s, func)(args);
}
}
auto reduce(alias func)() {
return ss[]
.map!(t => t.reduce!func)
.reduce!(func);
}
}
void main() {
auto test = new Container();
test.dispatch!"foo"(10);
test.dispatch!"foo"(20);
test.dispatch!"foo"(30);
test.dispatch!"bar"();
//auto x = test.reduce!((a, b) => a + b);
auto x = test.reduce!((int a, int b) => a + b); // Compiles
writeln(x);
}
Another way would be to take the delegate of one instance and to
patch the pointer to the instance for each instance (because the
member of a delegate can be changed: .ptr -> instance, .funcptr
-> static address of the function). But it's less clean.
2) This is about the reduce templates. As I've commented, I
can't use a template lambda with reduce, but I can use a lambda
taking ints as arguments. Why is this? The error message I get
when using the template lambda is:
"template instance reduce!((a, b) => a + b) cannot use local
'__lambda1' as parameter to non-global template reduce(alias
fun)()"
No idea for this.
