Re: struct opEquals does not work with parameter of same type - bug or feature?

[Jonathan M Davis]

On Monday, August 29, 2011 22:41:26 Sean Eskapp wrote:
 I am trying to build a struct with equality testing, using this code:
 
 struct Foo
 {
 const bool opEquals(Foo f)
 {
 return true;
 }
 }
 
 This gives me the error that the parameter should be of type ref const
 Foo. Fine.
 
 struct Foo
 {
 const bool opEquals(ref const Foo f)
 {
 return true;
 }
 }
 
 This, however, does not work with code like:
 
 Foo bar()
 {
 return Foo();
 }
 
 assert(Foo() == bar());
 
 function Foo.opEquals(ref const const(Foo) f) const is not callable using
 argument types (Foo) and bar() is not an lvalue.
 
 How can I do this?

http://d.puremagic.com/issues/show_bug.cgi?id=3659

http://stackoverflow.com/questions/6986175/const-ref-and-rvalue-in-d

- Jonathan M Davis

Re: struct opEquals does not work with parameter of same type - bug or feature?

[Sean Eskapp]

== Quote from Jonathan M Davis (jmdavisp...@gmx.com)'s article
 On Monday, August 29, 2011 22:41:26 Sean Eskapp wrote:
  I am trying to build a struct with equality testing, using this code:
 
  struct Foo
  {
  const bool opEquals(Foo f)
  {
  return true;
  }
  }
 
  This gives me the error that the parameter should be of type ref const
  Foo. Fine.
 
  struct Foo
  {
  const bool opEquals(ref const Foo f)
  {
  return true;
  }
  }
 
  This, however, does not work with code like:
 
  Foo bar()
  {
  return Foo();
  }
 
  assert(Foo() == bar());
 
  function Foo.opEquals(ref const const(Foo) f) const is not callable using
  argument types (Foo) and bar() is not an lvalue.
 
  How can I do this?
 http://d.puremagic.com/issues/show_bug.cgi?id=3659
 http://stackoverflow.com/questions/6986175/const-ref-and-rvalue-in-d
 - Jonathan M Davis

Ah, thanks!

Re: struct opEquals

[Steven Schveighoffer]

On Wed, 09 Mar 2011 11:40:25 -0500, SiegeLord n...@none.com wrote:


1) Why does this code not work (dmd 2.051) and how do I fix it:

struct S
{
static S New()
{
S s;
return s;
}

const bool opEquals(ref const(S) s)
{
return true;
}
}

void main()
{
S s;
assert(s == S.New);
}


Because passing an argument via ref means it must be an lvalue.  New()  
returns an rvalue.


However, that restriction is lifted for the 'this' parameter, so the  
following should actually work:


assert(S.New() == s);



2) Why is the type of struct opEquals have to be const bool opEquals(ref  
const(T) s)? Why is it even enforced to be anything in particular (it's  
not like there's an Object or something to inherit from)?


It's a mis-designed feature of structs.  There is a bug report on it:

http://d.puremagic.com/issues/show_bug.cgi?id=3659

-Steve

Re: struct opEquals

[SiegeLord]

Steven Schveighoffer Wrote:

 It's a mis-designed feature of structs.  There is a bug report on it:
 
 http://d.puremagic.com/issues/show_bug.cgi?id=3659

It worked fine in D1. Or did you mean that the mis-designed feature is the 
const system?

Anyway, thanks for the link to the bug report. I'll work around it for now.

-SiegeLord

 
 -Steve

Re: struct opEquals

[Steven Schveighoffer]

On Wed, 09 Mar 2011 12:15:26 -0500, SiegeLord n...@none.com wrote:


Steven Schveighoffer Wrote:


It's a mis-designed feature of structs.  There is a bug report on it:

http://d.puremagic.com/issues/show_bug.cgi?id=3659


It worked fine in D1. Or did you mean that the mis-designed feature is  
the const system?


No, the mis-designed feature is the compiler requiring that specific  
signature in D2.  Const is not the mis-designed feature.


It works in D1 because D1 doesn't generate intelligent opEquals for  
structs that do not have them, it just does a bit compare.


For example, if you did this in D1, it fails the assert:

struct S
{
   string name;
}

void main()
{
   S s1, s2;
   s1.name = hello.dup;
   s2.name = hello.dup;
   assert(s1 == s2); // fails in D1, should pass on D2.
}

-Steve