If you have a known class type string, like for example @"NSString", you would create an instance of that class like so:
Class stringClass = NSClassFromString(@"NSString");
NSString* myString = [[stringClass alloc] init];
But what if you don't know the type of the class that you receive? I mean, you know it will be a valid one, you just don't know which one ...
Is it still possible to create an instance of that class, not id?
If you are calling [[unknownClass alloc] init] you will always create the actual class. You're only saving the class in an id typed variable. But the class itself remains in its type. You could also store a string in an id variable, but still use the string methods. E.g.:
id string = @"lalalala";
CGSize size = [string sizeWithFont: [UIFont systemFontOfSize: 12]];
Does this answer your question?
performSelector dance, and in fact it distracts from your point that id types can be messages just as any other type.