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digitalmars.D - Why I can't call toHash in const context?

reply "Borislav Kosharov" <bosak gmail.com> writes:
I have this code:

class Animation {
     string name;
}

struct Key {
     Animation from;
     Animation to;

     this(Animation from, Animation to) {
         this.from = from;
         this.to = to;
     }

     const hash_t opHash() {
         return from.toHash() ^ to.toHash();
     }

     const bool opEquals(ref const Key other) {
         return this.from == other.from && this.to == other.to;
     }
}

One class Animation and another Key. And I get this error:
"opHash.d(15): Error: mutable method object.Object.toHash is not 
callable using a const object"
I need Key to be a struct and I need it to have toHash for 
associative arrays.

I looked in the docs and it said that I need 'const hash_t 
opHash' to do that.

Am I missing something?
Aug 06 2013
parent reply "Peter Alexander" <peter.alexander.au gmail.com> writes:
On Tuesday, 6 August 2013 at 18:14:29 UTC, Borislav Kosharov 
wrote:

 Am I missing something?


It's a long story :-)

Issue 1824 - Object not const correct
http://d.puremagic.com/issues/show_bug.cgi?id=1824

Issue 9771 - Remove toHash from Object
http://d.puremagic.com/issues/show_bug.cgi?id=9771

http://forum.dlang.org/thread/jtlj1k$1fdj$1 digitalmars.com#post-jtlj1k:241fdj:241:40digitalmars.com
Aug 06 2013
next sibling parent "Borislav Kosharov" <bosak gmail.com> writes:
On Tuesday, 6 August 2013 at 18:18:43 UTC, Peter Alexander wrote:

 On Tuesday, 6 August 2013 at 18:14:29 UTC, Borislav Kosharov 
 wrote:

 Am I missing something?


 It's a long story :-)

 Issue 1824 - Object not const correct
 http://d.puremagic.com/issues/show_bug.cgi?id=1824

 Issue 9771 - Remove toHash from Object
 http://d.puremagic.com/issues/show_bug.cgi?id=9771

 http://forum.dlang.org/thread/jtlj1k$1fdj$1 digitalmars.com#post-jtlj1k:241fdj:241:40digitalmars.com


I don't have much time to read all that, but from what I have 
skimmed I should remove the const attribute and it should work. 
And it is considered removing toHash from object?

Thanks for the info. I will read the links later.
Aug 06 2013
prev sibling parent reply "Borislav Kosharov" <bosak gmail.com> writes:
On Tuesday, 6 August 2013 at 18:18:43 UTC, Peter Alexander wrote:

 On Tuesday, 6 August 2013 at 18:14:29 UTC, Borislav Kosharov 
 wrote:

 Am I missing something?


 It's a long story :-)

 Issue 1824 - Object not const correct
 http://d.puremagic.com/issues/show_bug.cgi?id=1824

 Issue 9771 - Remove toHash from Object
 http://d.puremagic.com/issues/show_bug.cgi?id=9771

 http://forum.dlang.org/thread/jtlj1k$1fdj$1 digitalmars.com#post-jtlj1k:241fdj:241:40digitalmars.com


Also what is the way of calculating hashes for basic types like 
int and string. Those types don't inherit from Object, right? Is 
there some std function like hash(T)(T value) that is universal 
or something?
Aug 06 2013
next sibling parent "Peter Alexander" <peter.alexander.au gmail.com> writes:
On Tuesday, 6 August 2013 at 18:44:52 UTC, Borislav Kosharov 
wrote:

 Also what is the way of calculating hashes for basic types like 
 int and string. Those types don't inherit from Object, right?


Correct.


 Is there some std function like hash(T)(T value) that is 
 universal or something?




Use like so:

T value;
auto hash = typeid(value).getHash(&value);
Aug 06 2013
prev sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Tue, Aug 06, 2013 at 08:44:51PM +0200, Borislav Kosharov wrote:

 On Tuesday, 6 August 2013 at 18:18:43 UTC, Peter Alexander wrote:

On Tuesday, 6 August 2013 at 18:14:29 UTC, Borislav Kosharov
wrote:

Am I missing something?


It's a long story :-)

Issue 1824 - Object not const correct
http://d.puremagic.com/issues/show_bug.cgi?id=1824

Issue 9771 - Remove toHash from Object
http://d.puremagic.com/issues/show_bug.cgi?id=9771

http://forum.dlang.org/thread/jtlj1k$1fdj$1 digitalmars.com#post-jtlj1k:241fdj:241:40digitalmars.com


 
 Also what is the way of calculating hashes for basic types like int
 and string. Those types don't inherit from Object, right? Is there
 some std function like hash(T)(T value) that is universal or
 something?


Currently, the most reliable way to get hashes for basic types that also
matches what AA's use, is to do this:

	int x = 123;	// or, int[] x = [1,2,3], etc.
	auto hash = typeid(x).getHash(&x);

I say "currently" because this is likely to change in the future when we
clean up the AA implementation.


T

-- 
War doesn't prove who's right, just who's left. -- BSD Games' Fortune
Aug 06 2013