On Thursday, 11 October 2018 at 21:57:00 UTC, Jonathan M Davis
wrote:
On Thursday, October 11, 2018 1:09:14 PM MDT Jonathan Marler
via Digitalmars-d wrote:
On Thursday, 11 October 2018 at 14:35:34 UTC, James Japherson
wrote:
> Took me about an hour to track this one down!
>
> A + (B == 0) ? 0 : C;
>
> D is evaluating it as
>
> (A + (B == 0)) ? 0 : C;
>
>
> The whole point of the parenthesis was to associate.
>
> I usually explicitly associate precisely because of this!
>
> A + ((B == 0) ? 0 : C);
>
> In the ternary operator it should treat parenthesis directly
> to the left as the argument.
>
> Of course, I doubt this will get fixed but it should be
> noted so other don't step in the same poo.
In c++ the ternary operator is the second most lowest
precedence operator, just above the comma. You can see a
table of each operator and their precendence here, I refer to
it every so often:
https://en.cppreference.com/w/cpp/language/operator_precedence
Learning that the ternary operator has such a low precedence
is one of those things that all programmers eventually run
into...welcome to the club :)
It looks like D has a similar table here
(https://wiki.dlang.org/Operator_precedence). However, it
doesn't appear to have the ternary operator in there. On that
note, D would take it's precedence order from C/C++ unless
there's a VERY good reason to change it.
The operator precedence matches in D. Because in principle, C
code should either be valid D code with the same semantics as
it had in C, or it shouldn't compile as D code, changing
operator precedence isn't something that D is going to do
(though clearly, the ternary operator needs to be added to the
table). It would be a disaster for porting code if we did.
- Jonathan M Davis
I had a look at the table again, looks like the ternary operator
is on there, just called the "conditional operator". And to
clarify, D's operator precedence is close to C/C++ but doesn't
match exactly. This is likely a result of the grammar
differences rather than an intention one. For example, the
"Conditional operator" in D actually has a higher priority than
an assignment, but in C++ it's the same and is evaluated
right-to-left. So this expression would be different in C++ and
D:
a ? b : c = d
In D it would be:
(a ? b : c ) = d
And in C++ would be:
a ? b : (c = d)
Check it out:
---
import core.stdc.stdio;
void main()
{
int a = 2, b = 3;
printf("expr = %d\n", 1 ? a : b = 4);
}
---
prints "expr = 4"
it evaluates the conditional (1 ? a : b) into the expression a,
which is actually an rvalue! and then assigns it to 4 because of
the "= 4" and then returns the value 4 to printf.
Here's the C++ version:
#include <stdio.h>
int main(int argc, char *argv[])
{
int a = 2, b = 3;
printf("expr = %d\n", 1 ? a : b = 4);
}
This one prints "expr = 2"
It simply returns the value of `a` because the "b = 4" at the end
is all part of the "else" contition in the ternary operator.
