Re: Forward declaration of cyclical covariant types?

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 24 Jan 2009 21:15:12 CST
Message-ID:
<gldg5u\$5d7\$1@aioe.org>
* Patrik Kahari:

//file 1:
struct Y; //fwd decl

struct X {
virtual Y& covariantY();
};

struct Y {
virtual X& covariantX();
};

//file 2:
struct Y1; //fwd decl
//struct Y1: public Y; //How do you fwd declare that a type is derived
from another type?

struct X1: public X {
virtual Y1& covariantY(); //Error 'X1::covariantY': overriding
virtual function return type differs and is not covariant from
'X::covariantY
};

struct Y1: public Y {
virtual X1& covariantX();
};

<code>
// Module 1.

class Y;

class X
{
private:
virtual Y& virtualY();
public:
Y& publicY();
};

class Y
{
private:
virtual X& virtualX();
public:
X& publicX();
};

Y& X::virtualY() { static Y y; return y; }
Y& X::publicY() { return virtualY(); }
X& Y::virtualX() { static X x; return x; }
X& Y::publicX() { return virtualX(); }

// Module 2:
class Y1;

class X1: public X
{
private:
virtual Y& virtualY();
public:
Y1& publicY1();
};

class Y1: public Y
{
private:
virtual X& virtualX();
public:
X1& publicX1();
};

Y& X1::virtualY() { static Y1 y; return y; }
Y1& X1::publicY1() { return static_cast<Y1&>( virtualY() ); }
X& Y1::virtualX() { static X1 x; return x; }
X1& Y1::publicX1() { return static_cast<X1&>( virtualX() ); }

// Module 3.

int main()
{
X* p = new X1();

p->publicY(); // Reference to an Y1.
delete p;
}
</code>

But I suspect that it would be better to focus on fixing the design rather
than
finding ways to shoehorne the thing into C++ rules.

Also, don't take the static_cast's too literally.

Cheers & hth.,

- Alf

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