C++ Template Covariance

Question

Can you use the covariance propriety for the generic types (through the templates) in C++?

I already found this question that answers my question, but I ask it again since it has already been two years! In addiction, though it is explained that there can be no covariance in C++ in templates, there is no explanation about that!

Can you help me about news/explanation of this topic?

You should add a code sample for what you call "template covariance" : the answer of the linked question still apply — quantdev, Sep 03 '14 at 19:09
If some purported construction isn't part of the language, how much "explanation" would satisfy you regarding its absence from the language? Isn't "it doesn't exist" enough? — Kerrek SB, Sep 03 '14 at 19:11
"In addiction..." typo, or just can't get enough coding? Hmmm... — WhozCraig, Sep 03 '14 at 19:19
Actually, what you asks about in the linked question is named "contravariance", not "covariance" — Piotr Skotnicki, Sep 03 '14 at 19:25
@kerrek The C++ language is well-known for trying hard to encompass a lot of paradigms, so "it doesn't exists" is totally unsatisfying in this land. — Johan Boulé, Jul 27 '18 at 22:53

score 5 · Accepted Answer · answered Sep 03 '14 at 19:24

Given the reference to an earlier question as a clarification device, it seems you are asking why T<Derived> is not usually derived from T<Base>.

Consider T = std::shared_ptr.

You don't want to be able to do this:

void foo( shared_ptr<Base>& p ) { p.reset( new Derived2 ); }

auto main() -> int
{
    shared_ptr<Derived1> p;
    foo( p );   // Oops, p now points to unrelated Derived2
}

C++ Template Covariance

1 Answers1