I think putting it that way exaggerates the differences a bit. The message doesn...

pcwalton · on Aug 5, 2015

The vtable is a flat structure with known layout in C++. In Objective-C, it's a hash table (with precomputed hashes and lots of other tricks—but it's still fundamentally a hash table). There's a big difference between the two.

chc · on Aug 5, 2015

There's a big difference between the data structures, yes, but the basic logic for message sends is still pretty similar. They both boil down to a simple lookup in a data structure and a jump. Would you really characterize the difference between a vtable and a hash table as "you actually pass a message, that first must be interpreted" in the case of a hash table?

_3u10 · on Aug 5, 2015

I think the biggest issue is that in a vtable you jump directly to the method by index, and with objc_msgsend it looks up a string in a hash table.

Mostly the difference is ints vs. strings in my opinion.

As well in C++ if you don't mark your method virtual you can bypass all the vtable crap.

I love ObjC but method resolution is far more expensive than in C++, especially non-virtual methods.

astrange · on Aug 5, 2015

Method selectors in Obj-C don't have to be strings, as long as you can turn a string into one occasionally.

You do still need to do a hash lookup to get the method implementation, since the selector table changes at runtime and objects have to safely respond to unhandled selectors and all that.

_3u10 · on Aug 5, 2015

I meant string as in char* not NSString, selectors specifically the SEL datatype are strings

They certainly don't have to be, however, they are.

    SEL selector = @selector(applicationDidBecomeActive:);
    printf("%s",(char*)(void*)selector);
    // Prints applicationDidBecomeActive: