Why is there no parameter contra-variance for overriding?

Question

C++ and Java support return-type covariance when overriding methods.

Neither, however, support contra-variance in parameter types - instead, it translates to overl

Answer 1

For C++, Stroustrup discusses the reasons for hiding briefly in section 3.5.3 of The Design & Evolution of C++. His reasoning is (I paraphrase) that other solutions raise just as many issues, and it has been this way since C With Classes days.

As an example, he gives two classes - and a derived class B. Both have a virtual copy() function which takes a pointer of their respective types. If we say:

A a;
B b;
b.copy( & a );

that is currently an error, as B's copy() hides A's. If it were not an error, only the A parts of B could be updated by A's copy() function.

Once again, I've paraphrased - if you are interested, read the book, which is excellent.