[kragen]

The specific thing you can do in Smalltalk (or Ruby, Python, Objective-C, Erights E, or JS) that you can't do in C++ (even Qt C++, and not Simula either) is define a proxy class you can call arbitrary methods on, so that it can, for example, forward the method call to another object across the network, or deserialize an object stored on disk, or simply log all the methods called on a given object.

This is because, conceptually, the object has total freedom to handle the message it was sent however it sees fit. Even if it's never heard of the method name before.

[bluGill]

You can do that in C++ too - it is just a lot of manual work. Those other languages just hide (or make easy) all the work needed to do that. There are trade offs though - just because you can in C++ doesn't mean you should: C++ is best where the performance cost of that is unacceptable.

[kragen]

No, in C++ it's literally impossible. The language provides no way to define a proxy class you can call arbitrary methods on. You have to generate a fresh proxy class every time you have a new abstract base class you want to interpose, either by hand, with a macro processor, or with run-time code generation. There's no language mechanism to compile code that calls .fhqwhgads() successfully on a class that doesn't have a .fhqwhgads() method declared.

[bluGill]

you don't call fhqwhgads() on your proxy class though. You call runFunction("fhqwhgads") and it all compiles - the proxy class then string matches on the arguments. Of course depending on what you want to do it can be a lot more complex. That is do manually what other languages do for you automatically under the hood.

Again, this is not something you should do, but you can.

[kragen]

That doesn't provide the same functionality, because it requires a global transformation of your program, changing every caller of .fhqwhgads() (possibly including code written by users of your class that they aren't willing to show you, example code in the docs on your website, code in Stack Overflow answers, code in printed books, etc.). By contrast, in OO languages, you typically just define a single method that's a few lines of code.

You're sinking into the Turing Tarpit where everything is possible but nothing of interest is easy. By morning, you'll be programming in Brainfuck.