• biozic (15/15) Apr 02 2015 The code below doesn't compile. Why this error message?
• Nicholas Wilson (14/29) Apr 02 2015 this doesn't work because struct Foo is parameterised by in

"biozic" <dransic gmail.com> writes:

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

"Nicholas Wilson" <iamthewilsonator hotmail.com> writes:

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) { ... }