Comment by speeder | Hacker News Reader

Comment by speeder | Hacker News Reader

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Or explaining this better:

The objects of ObjC work very differently than C++ objects.

In Obj-C for example, there are no "methods" like C++, in C++ when you put a method in a class, (although implementation details may vary) it works like a function pointer in a C struct, and calling that method is just executing whatever code that pointer points to.

In Obj-C in contrast, to call a function of a class, you actually pass a message, that first must be interpreted, then the runtime that interpreted the message find the correct function definition to use and run.

For most apps people don't care about this, but game developers for example frequently make iOS and Mac games that in time-critical parts use pure C or even Obj-C++ so that they can skip this messaging part (that can cause significant performance problems sometimes).

Example: yesterday I was trying to figure why a open source game I like was slowing down, I ran a profiler, and the most called function, and the one with most execution time, was a simple "GetSomeRandomVar()" for spaceships (it is a spacefight battle game), I looked in the source, and saw that every frame, every spaceship call this at least once for every other spaceship, if the spaceship has an AI it calls it many, many times, I inlined it and the code became much faster.

I am very sure that if it was a ObjC call (with messaging) it would be so slow that the original author probably would not even have uploaded that to github. (by the way, the game IS an OSX game, but the original author wisely decided to go with C++ instead of ObjC)

I think putting it that way exaggerates the differences a bit. The message doesn't really have to be "interpreted" in any meaningful way AFAIK. A message consists of a selector (essentially a method name) and a receiver, then whatever other arguments the method will take. The runtime looks up the selector in the receiver's method table, much like with C++ virtual methods, and then jumps to the implementation it finds. There is a little more fanciness (some method caching, and support for nil receivers), but I don't feel like it's hugely dissimilar from virtual methods.

For anyone interested, here's a good analysis of Objective-C's message dispatch machinery: http://blog.zhengdong.me/2013/07/18/a-look-under-the-hood-of...

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.

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?

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.

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.

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:

> in C++ when you put a method in a class, (although implementation details may vary) it works like a function pointer in a C struct

That's true only for virtual methods.

Also to add to what you said, no real way of doing stack allocated objects in ObjC is an issue as well.

Also, since classes are mutable, calling a method in a multithreaded environment may result in lock contention, which I imagine being tricky in a driver.

So using obj-c in the kernel might require giving up some of the language's dynamism. Not sure how this was handled in NeXT.

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