On 5/11/17 7:52 PM, H. S. Teoh via Digitalmars-d wrote:
On Thu, May 11, 2017 at 07:46:24PM -0400, Steven Schveighoffer via
Digitalmars-d wrote:
[...]
But this still doesn't mean that *all* bool conversions are value
based. In at least the struct and class cases, more than just the bits
are checked.
[...]
Wait, what? You can use a *struct* as a bool condition?!
I tried this:
import std.stdio;
struct S {}
void main() {
S s;
if (s) { writeln("WAT"); }
}
But the compiler (rightly) said:
test.d(5): Error: expression s of type S does not have a boolean value
Or were you talking about structs that define opCast!bool? (In which
case it's certainly intentional and doesn't pose a problem.)
Yes, I was talking about that.
I can see classes being usable in conditions, though, since they're
essentially pointers hiding behind an abstraction. Still, it doesn't
quite sit right with me. For example:
class C { }
class D { bool opCast(T : bool)() { return false; } }
void main() {
C c;
D d = new D;
if (!c) { ... } // OK, expected semantics
if (!d) { ... } // *** What happens here?
}
I have totally misremembered the thing of classes and invariants. So my
statement on that is wrong.
It's when you *assert* a class instance that the invariant is checked,
not when used in an if condition.
Testing out your question, the opCast to bool doesn't apply for classes.
Just the reference is checked against null. You have to write
cast(bool)d to trigger the opCast.
So classes are just like pointers. Really only structs offer some
semblance of control for use in if statements.
-Steve
