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Polymorphic copy: Difference between revisions
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→{{header|Objective-C}}: modernize
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=={{header|Objective-C}}==
All objects inherit the <code>copy</code> method from <code>NSObject</code>, which performs copying. But they must implement the <code>NSCopying</code> protocol (which involves implementing the <code>copyWithZone:</code> method) to actually specify how to copy. Calling <code>copy</code> on an object that does not implement <code>copyWithZone:</code> will result in an error.
Generally, to implement copying, if your parent class does not implement <code>NSCopying</code>, you explicitly allocate a new object (using the class object <code>[self class]</code> instead of hard-coding the name of a particular class, in order to be polymorphic), and initialize it with your object's data. If your parent class already implements <code>NSCopying</code>, and you wish to customize the copying of your class's fields, then you should get a copy from your parent object's <code>copyWithZone:</code> method, and then perform custom initialization on the copy.
<lang objc>@interface T : NSObject
- (void)identify;
@end
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@interface S : T
@end
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int main()
{
@autoreleasepool {
T *original = [[S alloc] init];▼
T *another = [original copy];▼
▲ T *original = [[S alloc] init];
[another identify]; // logs "I am an S"▼
▲ T *another = [original copy];
▲ [another identify]; // logs "I am an S"
}
return 0;
}</lang>
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