Le 14/12/10 16:50, Andrei Alexandrescu a écrit :
On 12/14/10 7:33 AM, biozic wrote:
Le 14/12/10 09:56, Andrei Alexandrescu a écrit :
On 12/14/10 2:23 AM, Walter Bright wrote:
bearophile wrote:
Andrei:
D's constrained templates were introduced on 17 June 2008 and
they've
been
a smashing hit. Virtually all of Phobos uses them, and I'm sure a
lot of
generic client code. They take half a minute to explain and solve a
difficult problem in a very simple manner.<
A template with two constrains makes it less handy to add error
messages that
explain why the instantiation has failed:
Adding such error messages misses the point of constraints. The
idea is
that template overloading is based on constraints. Given 3 templates
with 3 constraints, overload resolution picks the one with constraints
that pass. Giving error messages for the others is wrong.
If you want it to produce error messages if the instantiation is
wrong,
the correct way is to use static asserts within the template body. No
additional features serve any purpose.
Yah, I was planning on writing the same response.
Andrei
I have a question about this and some pieces of code in the standard
library, notably std.algorithm: some of the templated functions use
template constraints even when no template overloading is taking place.
Wouldn't some static asserts help print more accurate messages when
these functions are misused?
Nicolas
The intent is to allow other code to define functions such as e.g.
"map" or "sort". Generally any generic function should exclude via a
constraint the inputs it can't work on. That way no generic function
bites off more than it can chew.
Andrei
Yes, I understand that this is more important than the accuracy of error
messages.
Thanks
