On Sunday, October 11, 2015 22:21:55 H. S. Teoh via Digitalmars-d-learn wrote:
> It's best to parenthesize when mixing other operators with ?, because ? has a pretty low precedence and may "steal" arguments from surrounding operators that you don't intend.  My suspicion is that what you wrote is being parsed as:
>
>   writeln(("foo " ~ true) ? "bar" : "baz");
>
> which is why you're getting unexpected output. Write instead:
>
>   writeln("foo " ~ (true ? "bar" : "baz"));
>
> If anything, it also helps readers of your code understand what it does without needing to consult the precedence table.

The ternary operator is on the next to bottom rung on the same level with the various assigment operators. The _only_ operator with lower precedence than the ternary operator is the comma (be it the comma operator or the comma as an argument separator). So, if you're doing something like

auto i = foo == bar ? "hello" : "world";

or

foo(arg1, foo == bar ? "hello" : "world", arg3);

then the ternary is done before the stuff around it, but other than that, everything around it is going to be done first. So, whether you need parens are not is actually pretty straightforward. It pretty much boils down to if you want the ternary to be done before anything else around it, use parens. If you want the ternary operator to be done last, then you don't need them.

I actually think that the ternary operator is very easy to get right because it's pretty much at one end of the operator precedence table rather than in the middle where you have to remember what goes before and what comes after. But for some reason, a lot of folks seem to assume that it has different precedence than it has and have trouble with it.

- Jonathan M Davis