On Monday, 8 June 2020 at 07:35:12 UTC, adnan338 wrote:
Hi, as far as I understand, the `this` template parameter
includes constness qualifiers as seen in
https://ddili.org/ders/d.en/templates_more.html
To apply this I have this following struct:
module bst;
struct Tree(T) {
T item;
Tree!T* parent, left, right;
this(T item) {
this.item = item;
}
Self* searchTree(this Self)(auto in ref T item) const {
if (&this is null)
return null;
if (this.item == item)
return &this;
return (this.item < item) ? this.right.searchTree(item)
: this.right.searchTree(item);
}
}
unittest {
auto text1 = "Hello", text2 = "World";
auto tree2 = Tree!string(text1);
assert(tree2.searchTree(text2) is null);
assert(tree2.searchTree(text1) !is null);
auto tree1 = Tree!int(4);
assert(tree1.searchTree(5) is null);
assert(tree1.searchTree(4) !is null);
}
When I run the unittest the compiler complains:
cannot implicitly convert expression &this of type
const(Tree!string)* to Tree!string*
Run link: https://run.dlang.io/is/b76DND
Why does this happen?
If I correctly understand what you are trying to do the answer is
- in D const is transitive (unlike the C++ where it isn't).
And in your searchTree() method you are basically trying to
escape that constness. You can change the signature to return
const(Self)*, or maybe add non-const overload depending on your
needs.
Don't even think about casting away const with `return
cast(Self*) &this;` as this UB in D, and it will bite you
somewhere later because compiler might rely on const for
optimization.
