Declaring a data type dynamically in C++

you would have to use unions to achieve something like that, but handling unions is a very difficile matter, so you should choose a container class which wraps the union logic behind an interface like Boost.Variant or Qts QVariant

You can't. This kind of run-time metaprogramming is not supported in C++.

The simple answer is that you can't - types need to be known at compile time in C++. You can do something like it using things like boost::any or unions, but it won't be pretty.

Everyone saying you can't do this in C++ is missing one obvious solution. This is where you could use a base class, you need to define the commonly used interface there, and then all the derived classes are whatever types you need. Put it in a smart pointer appropriate for a container and there you go. You may have to use dynamic type inference if you can't put enough of the interface in the base class, which is always frowned upon because it's ugly, but it's there for a reason. And dynamically allocating your types probably isn't the most efficient thing, but as always, it depends on what you're using it for.