Hey everyone.

I get an error on the following code complaining about the use of Box.toString. The message is:
"Error: cannot call mutable method on final struct"

I don't know what that means. I believe it's complaining about const stuff and the struct being altered (that is all that makes sense, given the message and the fact that it's in a loop), but looking at the toString method in std.boxer.Box, it doesn't seem to alter the struct. As far as I can tell, the code should be valid.

The workaround is using 'ref Box b' (or 'ref b') in the foreach loop.

---
import std.boxer, std.string, std.stdio;

class C(U...) {
   string value;
   this (U u) {
      value = "";
      foreach (Box b; boxArray(u)) {
         value ~= b.toString;
      }
   }
}

void main () {
   auto c = new C!(int, string, char)(12, "foom", 't');
   writefln(c.value);
}
---

Can anyone here else say whether this is a bug in Box.toString or in determining whether a function is mutable?

Christopher Wright escribió:
> Hey everyone.
> 
> I get an error on the following code complaining about the use of Box.toString. The message is:
> "Error: cannot call mutable method on final struct"
> 
> I don't know what that means. I believe it's complaining about const stuff and the struct being altered (that is all that makes sense, given the message and the fact that it's in a loop), but looking at the toString method in std.boxer.Box, it doesn't seem to alter the struct. As far as I can tell, the code should be valid.
> 
> The workaround is using 'ref Box b' (or 'ref b') in the foreach loop.
> 
> ---
> import std.boxer, std.string, std.stdio;
> 
> class C(U...) {
>    string value;
>    this (U u) {
>       value = "";
>       foreach (Box b; boxArray(u)) {
>          value ~= b.toString;
>       }
>    }
> }
> 
> void main () {
>    auto c = new C!(int, string, char)(12, "foom", 't');
>    writefln(c.value);
> }
> ---
> 
> Can anyone here else say whether this is a bug in Box.toString or in determining whether a function is mutable?

I don't use D 2.0, but here it goes...

string is const(char)[] or const char[] or something like that... It's const, anyway, so you aren't supposed to modify it. Replace "string value" with "char[] value".

-- 
Carlos Santander Bernal

Carlos Santander wrote:
> I don't use D 2.0, but here it goes...
> 
> string is const(char)[] or const char[] or something like that... It's const, anyway, so you aren't supposed to modify it. Replace "string value" with "char[] value".
> 

If I use char[] rather than string, it fails with the error message I stated (and indeed, in the original, I used char[] accidentally). If I just call Box.toString and don't do anything with the returned string, it fails with the same error message.

'string ~= whatever' is valid because the string is not final. The compiler rewrites it as 'string = string ~ whatever'.

New minimal test case:
---
import std.boxer;
void main () {
   foreach (b; boxArray(9, 'b')) {
      b.toString;
   }
}
---

"Christopher Wright" <dhasenan@gmail.com> wrote in message news:fc3bdu$8tn$1@digitalmars.com...
> 'string ~= whatever' is valid because the string is not final. The compiler rewrites it as 'string = string ~ whatever'.

Iiii wouldn't be so sure about that.  I don't know if the compiler rewrites "a ~= b" as "a = a ~ b" when dealing with const stuff, but as far as D1 goes, ~= and ~ are two different operations.  ~ always creates a copy of the original data, while ~= attempts to resize the destination in place and append the new data in the newly-resized space.

Christopher Wright wrote:

> New minimal test case:
> ---
> import std.boxer;
> void main () {
>     foreach (b; boxArray(9, 'b')) {
>        b.toString;
>     }
> }
> ---

The problem is that the b defined in the foreach statement is final, thus D won't let you call non-const methods on it. Boxer.toString is not a const method.

Same thing without boxer or toString:
---
struct Foo
{
  const void const_bar() {}
  void nonconst_bar() {}
}

void main()
{
  Foo[] foo_arr;
  foo_arr.length = 1;

  foreach(foo; foo_arr)
  {
    foo.const_bar(); // OK
    foo.nonconst_bar(); // Error: cannot call mutable method on final struct
  }
}
---

Christopher Wright escribió:
> Carlos Santander wrote:
>> I don't use D 2.0, but here it goes...
>>
>> string is const(char)[] or const char[] or something like that... It's const, anyway, so you aren't supposed to modify it. Replace "string value" with "char[] value".
>>
> 
> If I use char[] rather than string, it fails with the error message I stated (and indeed, in the original, I used char[] accidentally). If I just call Box.toString and don't do anything with the returned string, it fails with the same error message.
> 
> 'string ~= whatever' is valid because the string is not final. The compiler rewrites it as 'string = string ~ whatever'.
> 
> New minimal test case:
> ---
> import std.boxer;
> void main () {
>    foreach (b; boxArray(9, 'b')) {
>       b.toString;
>    }
> }
> ---

In that case, I guess the workaround (ref b) is not really a workaround but the way it should be done.

-- 
Carlos Santander Bernal