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From: gjditchfield@watmsg.waterloo.edu (Glen Ditchfield)
Newsgroups: comp.lang.c++
Subject: Re: typesafe downward casting and contravariance
Message-ID: <1991Apr10.145811.16061@watmath.waterloo.edu>
Date: 10 Apr 91 14:58:11 GMT
References: <9104091317.AA16916@ucbvax.Berkeley.EDU>
Sender: news@watmath.waterloo.edu (News Owner)
Organization: University of Waterloo
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In article <9104091317.AA16916@ucbvax.Berkeley.EDU> schweitz@lexvmc.vnet.ibm.com ("Eric Schweitz") writes:
> ...  Let's assume that the function sin() is contravariant...
>
> int sin (int);
> float sin (float);
> complex sin (complex);

That is not contravariance.  In contravariance, the types of arguments and
the return type change in the opposite way (hence the prefix "contra").

struct adam { ... };
struct bob: public adam {
    virtual bob* contra_f(bob*);
    virtual bob* co_f(bob*);
};

struct chuck: public bob {
    chuck* contra_f(adam*);         // contravariant redefinition
    chuck* co_f(chuck*);            // covariant redefinition
};