On Wednesday, 20 August 2014 at 21:06:49 UTC, monarch_dodra wrote:
On Wednesday, 20 August 2014 at 20:39:42 UTC, Jonathan M Davis
wrote:
is(typeof(foo)) and __traits(compiles, foo) are not the same.
The first tests for the existence of the symbol, whereas the
second checks whether the code will actually compile.
Is that even true? I mean, are you just repeating something
you've heard, or have you checked? "is(typeof(foo))" has always
failed for me merelly if "foo" fails to compile. "foo" being an
existing (but private) symbol is enough.
Test case:
//----
module foo;
struct S
{
private int i;
}
//----
import foo;
void main(string[] args)
{
S s;
writeln(is(typeof(s.i)));
writeln(__traits(compiles, s.i));
}
//----
This says false, false.
I've yet to find a usecase where "is(typeof(...))" and
"__traits(compiles, ...)" aren't interchangeable.
I mean, I may have missed something, but it seems the whole
"private" thing is just miss-information.
Well, hereas an example of them not being the same:
-------
import std.stdio;
struct S
{
static void foo()
{
writeln(is(typeof(this)));
writeln(__traits(compiles, this));
}
}
void main()
{
S.foo();
}
-------
I originally found out about it from Don here:
https://issues.dlang.org/show_bug.cgi?id=8339
I don't know why your example doesn't show them as different. But
we should probably change it so that they _are_ the same - either
that or document their differences explicitly and clearly, but I
don't know why the is(typeof.. behavior here would be desirable.
Maybe so that we could do type(this) to declare a local variable
of the class type generically? I don't know. They're _almost_ the
same but not quite, and I don't know what the exact differences
are. Pretty much all I have to go on is Don's explanation in that
bug report.
- Jonathan M Davis
