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.

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.