Re: Covariant return types and MI

From:
Joshua Maurice <joshuamaurice@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 5 Mar 2010 12:32:14 -0800 (PST)
Message-ID:
<4e17542a-0c1f-4369-b504-c008d41b1c3e@u5g2000prd.googlegroups.com>
On Mar 5, 10:37 am, Robert Fendt <rob...@fendt.net> wrote:

Hi all,

is this legal:

class interf1
{
  virtual ~interf1() {}
  virtual interf1* clone() = 0;

};

class interf2
{
  vitual ~interf2() {}
  virtual interf2* clone() = 0;

};

class impl : public interf1, public interf2
{
  impl* clone();

};

It seems to work at least in GCC 'pedantic ANSI' mode, and I
think I recall something about covariant return types being
explicitely allowed in standard C++. Cannot check with VC++
right now...

However, assuming that it is legal and well-defined: are there
any hidden snares I should be aware of?


Yes. Visual studios, all versions AFAIK do not support covariant
return types and multiple inheritance, or maybe virtual multiple
inheritance. I suggest the following quoted from Alf P. Steinbach here
http://groups.google.com/group/comp.lang.c++/msg/e50bd48283d94bb2
as a good workaround for this lack of support, though perhaps more
costly on compilers which actually support it. (I don't know
offhand.)

<code>
#include <string>
#include <iostream>

class base {
protected:
   virtual base * virtual_clone() const = 0;
public:
   virtual void do_x() { std::cout << "base::do_x()\n"; }
   virtual int * value() = 0;
   virtual ~base() {}
   base* clone() const { return virtual_clone(); }

};

class sub_base1
     : public virtual base
{
public:
   virtual void do_x() { std::cout << "sub_base1::do_x()\n"; }

};

class sub_base2
     : public virtual base
{
public:
   virtual void do_x() { std::cout << "sub_base2::do_x()\n"; }

};

class derived
     : public sub_base1
     , public sub_base2
{
protected:
   virtual base * virtual_clone() const { return new derived; }
public:
   virtual void do_x() { std::cout << "derived::do_x()\n"; }
   derived* clone() const
   { return dynamic_cast<derived*>( virtual_clone() ); }
   int * value() { return 0; }

};

class sub_base3
     : public sub_base1
{
public:
   void do_x()
   {
     std::cout << "sub_base3::do_x() -> " << *value() << "\n";
   }
   sub_base3* clone() const
   { return dynamic_cast<sub_base3*>( virtual_clone() ); }

};

class derived2
     : public sub_base3
{
protected:
   virtual base * virtual_clone() const { return new
derived2(*this); }
public:
   void do_y() {}
   derived2 * clone() const
   { return dynamic_cast<derived2*>( virtual_clone() ); }
   int * value() { return val; }

   derived2(int * v) : val(v) {}
private:
   int * val;

};

int main()
{
   int x = 5;
   derived2 d(&x);
   d.do_x();
   //d.virtual_clone()->do_x();
   base * b = &d;
   b->do_x();

   base * b2 = b->clone();
   b2->do_x();

   sub_base3 * sb3 = dynamic_cast<sub_base3*>(b2);
   sb3->do_x();
   sub_base3 * sb32 = sb3->clone(); // this call doesn't explode.
   sb32->do_x();
}

</code>

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