Polymorphism and vectors

[Anakreon]

I have a little problem with polymorphism and vectors.

Consider the example in the attached file

The output of the program is:
4base, 2b1, 2b2, 2b3
P4base
P4base
P4base

4base
4base
4base

Why are the descendants of class "base" are returned as instances of class "base"?
Is there a solution to this problem?

Thanks is advance,
Anakreon
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