Calling class methods via class name vs self

objective-c class-method
Milad picture Milad · Apr 29, 2012 · Viewed 26k times · Source

Suppose we have a class named Calculator. There's a class method in it, called runProgram. If I wanted to call this class method, inside the class's implementation, what would the difference between these two be:

[Calculator runProgram]

OR

[self runProgram]

Are these both the same?

Answer

user529758 picture user529758 · Apr 29, 2012

If inside an instance method:

[self runProgram]

in this, self means the object instance itself, and thus it will generate a runtime error. You want to use

[[self class] runProgram]

instead.

However, if you call this method from another class method, then

[self runProgram]

is correct, since now self refers to the class itself. I'd discourage to use

[Calculator runProgram]

because then subclasses of the class would erroneously call the superclass' (Calculator's) method instead of a possibly overridden method.

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