sort using delegate as predicate
The following code does not compile, because the custom predicate
of std.algorithm.sort is a template parameter, and therefore can
only be a function, but not a delegate. In C++, there is a
variant of sort taking a function-object as a second (run-time)
parameter, but phobos does not seems to have such a thing. It
seems like a pretty common problem to me, so does anyone have a
solution?
class Foo
{
int[] order;
bool cmp(int a, int b) const
{
return order[a] order[b];
}
}
void main()
{
auto f = new Foo;
int[] data;
sort!(f.cmp)(data);
}
Re: sort using delegate as predicate
On Tue, 16 Sep 2014 16:19:10 +, Simon Bürger wrote: The following code does not compile, because the custom predicate of std.algorithm.sort is a template parameter, and therefore can only be a function, but not a delegate. In C++, there is a variant of sort taking a function-object as a second (run-time) parameter, but phobos does not seems to have such a thing. It seems like a pretty common problem to me, so does anyone have a solution? Bit of a workaround, but this should do it: sort!((a,b) = f.cmp(a, b))(data);
Re: sort using delegate as predicate
On Tue, Sep 16, 2014 at 04:19:10PM +, via Digitalmars-d-learn wrote:
The following code does not compile, because the custom predicate of
std.algorithm.sort is a template parameter, and therefore can only be
a function, but not a delegate.
What makes you think so? Template parameters certainly can be delegates.
I use that all the time.
In C++, there is a variant of sort taking a function-object as a
second (run-time) parameter, but phobos does not seems to have such a
thing. It seems like a pretty common problem to me, so does anyone
have a solution?
class Foo
{
int[] order;
bool cmp(int a, int b) const
{
return order[a] order[b];
}
}
void main()
{
auto f = new Foo;
int[] data;
sort!(f.cmp)(data);
}
Try this:
sort!(f.cmp)(data);
In D, writing `f.cmp` (without the ) can in some contexts be
misinterpreted as calling the function and passing the return value to
the template parameter, which is probably why you're getting a compile
error.
T
--
Why ask rhetorical questions? -- JC
Re: sort using delegate as predicate
On Tuesday, 16 September 2014 at 16:27:46 UTC, H. S. Teoh via Digitalmars-d-learn wrote: On Tue, Sep 16, 2014 at 04:19:10PM +, via Digitalmars-d-learn wrote: The following code does not compile, because the custom predicate of std.algorithm.sort is a template parameter, and therefore can only be a function, but not a delegate. What makes you think so? Template parameters certainly can be delegates. I use that all the time. Well because the delegate contains a pointer to the object, which can not be known at compile time. In fact sort!(f.cmp)(data); fails with the error variable f cannot be read at compile time, which seems reasonable to me. On the other hand sort!((a,b) = f.cmp(a, b))(data); does in fact compile, so i guess problem is solved. Thanks guys.
Re: sort using delegate as predicate
On Tue, Sep 16, 2014 at 04:38:04PM +, via Digitalmars-d-learn wrote: On Tuesday, 16 September 2014 at 16:27:46 UTC, H. S. Teoh via Digitalmars-d-learn wrote: On Tue, Sep 16, 2014 at 04:19:10PM +, via Digitalmars-d-learn wrote: The following code does not compile, because the custom predicate of std.algorithm.sort is a template parameter, and therefore can only be a function, but not a delegate. What makes you think so? Template parameters certainly can be delegates. I use that all the time. Well because the delegate contains a pointer to the object, which can not be known at compile time. In fact sort!(f.cmp)(data); fails with the error variable f cannot be read at compile time, which seems reasonable to me. On the other hand sort!((a,b) = f.cmp(a, b))(data); does in fact compile, so i guess problem is solved. Thanks guys. Mea culpa, I confused lambdas with delegates. The two are not the same thing. That will teach me to answer emails while rushing to get ready for work. :-P Sorry for the noise. T -- There is no gravity. The earth sucks.
