Question

I’m learning D and have seen a lot of code like this:

ushort x = to!ushort(args[1]);

I assume this casts args[1] to ushort, but what’s the difference between this and cast(ushort)?

EDIT: And what other uses does the exclamation mark operator have?

Answer 1

In D,

to!ushort(args[1])

is shorthand for the template instantiation

to!(ushort)(args[1])

and is similar to

to<ushort>(args[1])

in languages like C++/Java/C#.

The exclamation point is to note the fact that it’s not a regular argument, but a template argument.

The notation does not use angle brackets because those are ridiculously difficult to parse correctly for a compiler (they make the grammar very context-sensitive), which makes it that much more difficult to implement a correct compiler. See here for more info.

The only other use I know about is just the unary ‘not’ operation (e.g. false == !true)… I can’t think of any other uses at the moment.

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Regarding the cast:

cast(ushort) is an unchecked cast, so it won’t throw an exception if the value is out of range.

to!ushort() is a checked cast, so it throws an exception if the value is out of range.