Re: Covaraint Parameters

"Alf P. Steinbach" <>
1 Nov 2006 17:49:28 -0500
* RenjithMohan:

Most modern C++ compilers support covariant return types.
What is there in the language which prevents covariant parameters?.

Rather it's a question of what isn't there, namely specification of
parameters as in, in/out or pure out.

Pure out-parameters can be covariant (in a type safe manner), and
likewise, pure in-parameters can be contra-variant (in a type safe manner).

In C++ only function return values are known to be pure out-direction,
and so covariance is only supported for function return values, and
there limited to pointers and references.

Depending on the type pointed to as well as the parameter passed the
appropriate override should be called, thereby removing the need for
convoluted patterns like the Visitor.
Thus I could have something like

class Context

class Context1 : public Context

class Base
public :
    virtual void f(Context* c)

class Derived : public Base
    void f(Context* c) //1
    void f(Context1* c) //2

This is ordinary overloading and has little or nothing to do with

int _tmain(int argc, _TCHAR* argv[])
    Base* p = new Derived;
    p->f(new Context); //calls 1
    p->f(new Context1); //also calls 1

    return 0;

Here what happens really is that the first overload is called always
regardles of the parameter type.

Why is this not possible to implement Covariant parameters?.

First of all, the above code example isn't relevant.

For the reason why covariance isn't generally possible in a type safe
manner in C++, consider implementing out-arguments, normally function
results (which can be covariant in C++), in this manner:

    struct Base { void sayBoo(); };
    struct Derived: Base { void sayNiceThings(); };

    struct Widget
        virtual void get( Base*& p ) const
            p->sayBoo(); p = 0;

    struct Button: Widget
        virtual void get( Derived*& p ) const
            p->sayNiceThings(); p = 0;

    int main()
       Widget const& w = Button();
       Base* p = new Base();

       w.get( p );

Now if Button::get was a covariant override of Widget:get, it would call
a non-existent member function on the object pointed to by p: the
compiler allows that call because there's nothing that designates the
parameter as an out-parameter, so the compiler must assume a valid
pointer can be passed in. Happily all that happens here is that Widget
says Boo!, perhaps scaring you a little. But, this being just after
Halloween, it's not all that scary. ;-)

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