On Thursday, 2 April 2015 at 23:12:25 UTC, biozic wrote:
The code below doesn't compile. Why this error message?
---
struct Item {
int i;
}
struct Params {
Item* item;
this(int i) {
item = new Item(i); // line 9
}
}
struct Foo(Params params) {}
enum foo = Foo!(Params(1));
---
test.d(9): Error: Item(1) is not an lvalue
this doesn't work because struct Foo is parameterised by in
instance of Params. (like if you were implementing a fixed size
array: struct FixedSizeArray(size_t len) {...}. here
FixedSizeArray is parameterised by in instance of a size_t).
Item is a pointer in Params and thus if it were to compile the
compiler would need to know the value returned by `new`at compile
time.
There are two ways to get that to compile change `Item* item;` to
`Item item;` or change `item = new Item(1);` to `item = null;`
tl;dr you can't have r-value (pointers) in instance template
parameter lists.
i.e. struct foo(my_reference_type_or_type_containg_pointers
instance) { ... }
