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From: Hyman Rosen <hymie@prolifics.com>
Subject: Re: Ada OO Mechanism
Date: 1999/05/26
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Ray Blaak <blaak@infomatch.com> writes:
> Well, here is an OO example showing difficulties with the C++ model
> as compared to Ada.
> 
> Ada:
> 
> generic
>    type ScalarType is (<>); -- Is this right? My Ada is rusty.
> package Vector is
>    type T is private;
> 
>    function Create(size : Natural) : T;
> 
>    function "*" (vector : in T; scalar : in ScalarType) : T;
>    function "*" (scalar : in ScalarType; vector : in T) : T;
> private
>    -- whatever
> end;
> 
> declare
>    package IntVector is new Vector(Integer); 
>    use type IntVector;
>    package FloatVector is new Vector (Float); 
>    use type FloatVector;
> 
>    I : IntVector.T := IntVector.Create(3);
>    F : FloatVector.T := FloatVector.Create(3);
> begin
>    I := I*2;
>    I := 2*I;
>    F := F*2;
>    F := 2*F;
> end;

OK, here's the C++, which compiles without error with EGCS:

template <typename ScalarType>
struct Vector
{
	explicit Vector(int n);
	Vector operator *(ScalarType scalar) const;
	friend Vector operator *(ScalarType scalar, const Vector &v)
	{
		return v * scalar;
	}
};

int main()
{
	Vector<float> f(3);
	Vector<int> i(2);

	f * 2;
	2 * f;

	i * 7;
	7 * i;
}

> In this example, things won't link because I "forgot" to declare the
> Vector<float> friend function.

By writing the function inside the class, I've wriiten it once for
all types with which Vector is instantiated.

> The point is the C++ is a little more clumsy in this regard. If anyone has a
> way to do the C++ example cleanly, I would be most interested

And that was pretty much my point. People who know one language better
than another will often think that the other language is incapable or
clumsy at certain things. Often, this is not the case.