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digitalmars.D.ide - Imcompatible type, const and const

reply "Duarte" <xporg.roso gmail.com> writes:
Hi guys,
While I was out there, trying some D and stuff, I came with an 
error message I can't understand.

Let's assume I have something like:

class Point2
{
     Vector2 opBinary(string op)(const Point2 _right)
     {
         static if(op == "-")
         {
             return new Vector2(   this.x - _right.x,
                                   this.y - _right.y   );
         }
     }

     public static float DistanceSq(const Point2 _left, const 
Point2 _right)
     {
         return _left.DistanceSq(_right);
     }

     public float Distance(const Point2 _right) const
     {
         Vector2 vec = this - _right;
         [...]
     }
}

The line "Vector2 vec = this - _right" is giving me the following 
error:
Error: incompatible types for ((this) - (_right)): 
'const(Point2)' and 'const(Point2)'

What am I missing here? The same code in C++ compile, so I was 
assuming it would just as well compile in D.

Cheers!
Mar 09 2014
next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Duarte:


 What am I missing here? The same code in C++ compile, so I was 
 assuming it would just as well compile in D.


C++ and D are two different languages, and among other things 
they have two very different constant-ness systems, so code in 
one language often can't work in the other.

In your code probably there are multiple problems. One problem I 
see is here:

     Vector2 opBinary(string op)(const Point2 _right) {
         static if(op == "-") {
             return new Vector2(this.x - _right.x, this.y - 
_right.y);
         }
     }

You have to use a template constraint otherwise one of your code 
paths returns nothing, and this is (thankfully) an error in D.

Also regarding your style, in D functions start with a lower case 
letter.

I suggest to ask similar questions in the D.learn newsgroup.

Bye,
bearophile
Mar 09 2014
parent reply "Duarte" <xporg.roso gmail.com> writes:
I don't know how I ended up in D.ide...

I did not copy all the code, because it didn't matter in this 
case. The path that returns nothing is actually a:
static if(op == "-")
{
     ...
}
else
{
     static assert(...)
}

Which hopefully won't compile if the operation is not supported.
Mar 09 2014
parent reply "Duarte" <xporg.roso gmail.com> writes:
On Sunday, 9 March 2014 at 16:56:52 UTC, Duarte wrote:

 I don't know how I ended up in D.ide...

 I did not copy all the code, because it didn't matter in this 
 case. The path that returns nothing is actually a:
 static if(op == "-")
 {
     ...
 }
 else
 {
     static assert(...)
 }

 Which hopefully won't compile if the operation is not supported.


Hmm
Vector2 vec = cast(Point2)this - cast(Point2)_right
does the trick. But I can't help to feel this is a ugly 
hack/workaround to a problem elsewhere...
Mar 09 2014
parent "bearophile" <bearophileHUGS lycos.com> writes:
Duarte:


 Hmm
 Vector2 vec = cast(Point2)this - cast(Point2)_right
 does the trick. But I can't help to feel this is a ugly 
 hack/workaround to a problem elsewhere...


In your code you don't have this problem, but in D, unlike C++, 
mutating const/immutable data is undefined, and leading to 
unpredictable but frequent bugs.

Bye,
bearophile
Mar 09 2014
prev sibling parent Mike Parker <aldacron gmail.com> writes:
On 3/10/2014 1:15 AM, Duarte wrote:

 Hi guys,
 While I was out there, trying some D and stuff, I came with an error
 message I can't understand.

 Let's assume I have something like:

 class Point2
 {
      Vector2 opBinary(string op)(const Point2 _right)
      {
          static if(op == "-")
          {
              return new Vector2(   this.x - _right.x,
                                    this.y - _right.y   );
          }
      }

      public static float DistanceSq(const Point2 _left, const Point2
 _right)
      {
          return _left.DistanceSq(_right);
      }

      public float Distance(const Point2 _right) const
      {
          Vector2 vec = this - _right;
          [...]
      }
 }

 The line "Vector2 vec = this - _right" is giving me the following error:
 Error: incompatible types for ((this) - (_right)): 'const(Point2)' and
 'const(Point2)'

 What am I missing here? The same code in C++ compile, so I was assuming
 it would just as well compile in D.

 Cheers!


I'm assuming that the error is occurring when you call the static 
DistanceSq. What's happening is that the static method takes two const 
Points. The call to _left.DistanceSq means that the this object for that 
call (_left) is const. That call passes because DistanceSq (which I 
assume you've mistyped as Distance here) is declared as a const method. 
The problem, though, is that your opBinary is not a const method so it 
cannot work on const(this) object -- const(this) - const(_right) is what 
you have. If you make opBinary a const method, you won't see this error 
anymore.

Also, it's probably a better idea to use structs for object types like 
vectors and points. D's GC isn't state of the art, so something as 
frequently allocated as points and vectors will likely cause issues.
Mar 09 2014