Re: toString() through interface

[Steven Schveighoffer via Digitalmars-d-learn]

On Sat, 19 Apr 2014 20:51:49 -0400, David Held <d...@wyntrmute.com> wrote:
On 4/19/2014 5:35 PM, David Held wrote:
interface Foo { }

class Bar : Foo
{
     override string toString() pure const { return "Bar"; }
}

void main()
{
     Foo foo = new Bar;
     foo.toString();
}

To make things more interesting, consider the call to toString() from  
inside a class (which is closer to my actual use case):

class Baz
{
     override string toString() pure const
     { return cast(Object)(foo).toString(); }

     Foo foo;
}

This really makes the compiler go bonkers:

src\Bug.d(11): Error: pure nested function 'toString' cannot access  
mutable data 'foo'
src\Bug.d(11): Error: pure nested function 'toString' cannot access  
mutable data 'foo'
src\Bug.d(11): Error: no property 'toString' for type 'Bug.Foo'
src\Bug.d(11): Error: pure nested function 'toString' cannot access  
mutable data 'foo'
src\Bug.d(11): Error: need 'this' for 'foo' of type 'Bug.Foo'

Apparently, DMD features "high-availability error reporting", because  
the first message might not be received by the programmer, so it writes  
it again, then gives you a different message, and writes it one more  
time, just in case there was a communication error or some other fault  
preventing you from receiving this important message about access to  
'foo'.

Again, the cast appears to do absolutely nothing, as the compiler  
insists on looking up toString() in Foo instead of Object.  What is  
really peculiar is that message 1, 2, and 4 are complaining that  
Baz.toString() is not allowed to access foo because it is mutable.  And  
yet, the 5th error message tells us how to fix it: just add 'this.':

     override string toString() pure const
     { return cast(Object)(this.foo).toString(); }

Now we just get this:

src\Bug.d(11): Error: no property 'toString' for type 'const(Foo)'


To explain what the compiler is having trouble with, you have to  
understand precedence.

cast(Object) does not come before '.'

So what the compiler thinks you said is:

return cast(Object)(this.foo.toString());

Others have said how to fix it. I just wanted to point out why you need to  
fix it that way.

-Steve