On Thursday, 3 October 2019 at 04:32:52 UTC, Brett wrote:
Ok, fine!
auto r = &x.a;
Now r is a reference, great!
No, r is a pointer, not a reference. D does not have reference
variables.
But now the semantics of using the array completely change and
errors abound!
Probably because you're trying to use r as if it were of type Y[]
when it's really of type Y[]*, i.e. if you wanted to append to it
you would need to first dereference the pointer:
*r ~= Y(10.0);
Normal D pointers tend to act very nice, e.g., we don't have to
use the C++ -> BS....
But with arrays it seems things are all screwed up and do not
work correctly.
No, they're working as intended.
I can't just use r like I used x.a. I can't even use *r as
things then get even more screwed up.
Appending to *r works as expected:
https://run.dlang.io/is/IQa619
What sort of trouble did you encounter?
I want to have a slice of an array that I can append to as a
sort of temporary and have it append to the main array(the end
of the slice always is the end of the main array)..
Your issue has nothing to do with structs.
As per the documentation:
https://dlang.org/spec/arrays.html#slicing
the initialization of b here creates a slice referencing a:
int[] a = [0, 1, 2];
int[] b = a;
As a slice, b has its own .ptr and .length properties that are
distinct from those of a. They are initialized with the values of
a's properties, but they *are not references* to a's properties
and b is *not a reference* to a.
assert(b.ptr == a.ptr);
assert(b.length == a.length);
Because b.ptr and a.ptr point to the same memory, any updates to
existing array elements through one will be reflected in the
other:
a[1] = 3;
writeln(b); // prints [0, 3, 2]
But any changes to the .ptr or .length of one will not be
reflected in the other:
a ~= 10;
assert(a.length == 4);
assert(b.length == 3);
writeln(a); // prints [0, 3, 2, 10];
writeln(b); // prints [0, 3, 2];
Though at this point, depending on whether or not a reallocation
took place, they might still be pointing to the same block of
memory.
I've tried assumeSafeAppend but it does nothing to help.
assumeSafeAppend is for telling the runtime that you want to
reuse an existing memory block backing an array and there's no
need to reallocate it in append.
https://dlang.org/articles/d-array-article.html#slice-members-appender
The reason is because i have an algorithm that generates data
and stores it in an array > and it is more efficient and easier
to code if I can just append data to a "slice" of the
array(there are no issues with overwriting).
Essentially it just acts as an alias.
By using pointers it would work but it screws up the entire
syntax of all the code and > then still doesn't work.
Could you adjust the signature of your algorithm to take the
array by ref?
void myAlgo(ref Y[] arr) {
arr ~= Y(10.2);
}
Or to take x by ref (or as a pointer) and manipulate the array
directly:
void myAlgo(ref X x) {
x.a ~= Y(10.2);
}
