Databases
StackOverflow2013
Postsdbo
POBest practices for overriding isEqual: and hash
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POBest practices for overriding isEqual: and hash
    primarykey
    data
    text
    <p>How do you properly override <code>isEqual:</code> in Objective-C? The "catch" seems to be that if two objects are equal (as determined by the <code>isEqual:</code> method), they must have the same hash value.</p> <p>The <a href="http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW25" rel="noreferrer">Introspection</a> section of the <a href="http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaFundamentals/" rel="noreferrer">Cocoa Fundamentals Guide</a> does have an example on how to override <code>isEqual:</code>, copied as follows, for a class named <code>MyWidget</code>:</p> <pre><code>- (BOOL)isEqual:(id)other { if (other == self) return YES; if (!other || ![other isKindOfClass:[self class]]) return NO; return [self isEqualToWidget:other]; } - (BOOL)isEqualToWidget:(MyWidget *)aWidget { if (self == aWidget) return YES; if (![(id)[self name] isEqual:[aWidget name]]) return NO; if (![[self data] isEqualToData:[aWidget data]]) return NO; return YES; } </code></pre> <p>It checks pointer equality, then class equality, and finally compares the objects using <code>isEqualToWidget:</code>, which only checks the <code>name</code> and <code>data</code> properties. What the example <em>doesn't</em> show is how to override <code>hash</code>.</p> <p>Let's assume there are other properties that do not affect equality, say <code>age</code>. Shouldn't the <code>hash</code> method be overridden such that only <code>name</code> and <code>data</code> affect the hash? And if so, how would you do that? Just add the hashes of <code>name</code> and <code>data</code>? For example:</p> <pre><code>- (NSUInteger)hash { NSUInteger hash = 0; hash += [[self name] hash]; hash += [[self data] hash]; return hash; } </code></pre> <p>Is that sufficient? Is there a better technique? What if you have primitives, like <code>int</code>? Convert them to <code>NSNumber</code> to get their hash? Or structs like <code>NSRect</code>?</p> <p>(<strong>Brain fart</strong>: Originally wrote "bitwise OR" them together with <code>|=</code>. Meant add.)</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USQuinn Taylor
    1. USDave Dribin
    plurals
    1. PL
      singulars
      1. LTLinked
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    3. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POBest practices for overriding isEqual: and hash
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POBest practices for overriding isEqual: and hash
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POBest practices for overriding isEqual: and hash
      1. This table or related slice is empty.
      1. VTUpMod
    1. CO`if (![other isKindOfClass:[self class]])` - This technically means equality will not be Commutative. I.e. A = B does not mean B = A (e.g. if one is a subclass of the other)
      singulars
      1. POBest practices for overriding isEqual: and hash
      1. USRobert
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload