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Liysh · Guest

According to Thinking in C++
"You cannot modify the return type of a virtual function during
overriding.but there is a special case in which you can slightly
modify the return type. If you=A1=AFre returning a pointer or a
reference to a base class, then the overridden version of the function
may return a pointer or reference to a class derived from what the base
returns." And here's the example:

class PetFood
{
public:
virtual string foodType() const = 0;
};

class Pet
{
public:
virtual string type() const = 0;
virtual PetFood* eats() = 0;
};

class Bird : public Pet
{
public:
string type() const { return "Bird"; }
class BirdFood : public PetFood
{
public:
string foodType() const
{
return "Bird food";
}
};

// Upcast to base type:
PetFood* eats() { return &bf; }
private:
BirdFood bf;
};

class Cat : public Pet
{
public:
string type() const { return "Cat"; }
class CatFood : public PetFood
{
public:
string foodType() const { return "Birds"; }
};
// Return exact type instead:
CatFood* eats() { return &cf; }
private:
CatFood cf;
};

int main()
{
Bird b;
Cat c;
Pet* p[] { &b, &c, };
for(int i = 0; i < sizeof p / sizeof *p; i++)
cout << p[i]->type() << " eats "
<< p[i]->eats()->foodType() << endl;
// Can return the exact type:
Cat::CatFood* cf = c.eats();
Bird::BirdFood* bf;
// Cannot return the exact type:
//! bf = b.eats();
// Must downcast:
bf = dynamic_cast }

What annoying me is that once I get code above compiled, I get a
compiler error saying
"overriding virtual function differs from 'Pet::eats' only by return
type or calling convention" which is for "CatFood* eats() { return &cf;}"
But this differ is author's intension, How's that was flagged as an
error?

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Bob Hairgrove · Guest

On 26 Nov 2005 09:27:38 -0500, "Liysh" <liysh (AT) intertimes (DOT) com.cn>
wrote:

Alberto Ganesh Barbati · Guest

Liysh wrote: