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Victor Bazarov
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 Posted: Fri Jul 25, 2003 2:27 am Post subject: Re: cannot compare base* to const derrived* |
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"Immanuel Scholz" <einwegadresse (AT) gmx (DOT) de> wrote...
| Quote: | Today, I tried the following statement and got an compiler error:
class Base {};
class Derrived : public Base {};
int main()
{
Base* b = 0;
const Derrived* d = 0;
return (b == d);
}
The compiler refuses to convert either Base* to const Derrived*
(downcast) or convert const Derrived* to Base* (constcast).
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Neither cast can be made implicitly, both require explicit casts.
However, 'b' can be converted to const Base* and 'd' can be
converted to const Base*, and both are accepted. That's what
the compiler should do.
What did the compiler say? I tried this on Comeau and it gladly
accepted the code.
| Quote: | But it could convert both, Base* to const Base* and const Derrived* to
const Base*.
Why refuses the compiler to resolve the casts? Is it an error within
the compiler?
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Very well could be.
| Quote: | I thought it is only forbidden to implicit convert one function
argument twice, but not to convert more than one argument only once?
Am I correct?
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Probably. Rules of overload resolution are so many and so...
How should I put it?... murky. At least to me. I remember that
for the assignment operator the left operand is not converted.
As to other conversions, I am blanking out for a moment.
| Quote: | If the compiler is right, is there a reasoning about why it is
forbidden?
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I think the compiler is wrong.
Victor
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