Why isn't SEL a class in Objective-C?

In Objective-C (at least, the Apple flavor of Obj-C), why is SEL not a class? Is it a matter of efficiency? Is it to prevent some sort of infinite recursion? Was there merely no incentive to make SEL a class? Speculation welcome, but plea开发者_开发知识库se let me know if an answer is historical truth or speculation.

It goes back to the earliest versions of Objective-C. If I could find my copy of the Objective-C book, I could give you a reference, but I think it just wasn't obvious what the benefit would be. The early versions of Objective-C were very minimal extensions of the C syntax.

In GCC, the SEL type was implemented as a const char *, pointing to a string representation of the selector's name. This implementation took advantage of constant string coalescing, which was already implemented in the compiler, to ensure uniqueness of selector values.

"The book" of course is referring to Object-Oriented Programming: An Evolutionary Approach. Thanks for jogging my memory Friedrich.

Making it a class type adds little or no benefit. If it were an NSObject subclass it would be a tremendous amount of overhead for something that is effectively supposed to be a unique identifier. To dispatch an Objective-C call at that point would incur a penalty such a perfomance critical section of code can't afford to pay.

And yes, I'm speculating as to justifications why the type is what it is.