DIP 1008 Preliminary Review Round 1

[Moritz Maxeiner via Digitalmars-d]

On Saturday, 20 May 2017 at 02:05:21 UTC, Jonathan M Davis wrote:
>
> 2. This really isn't going to fix the @nogc problem with 
> exceptions without either seriously overhauling how exceptions 
> are generated and printed or by having less informative error 
> messages. The problem is with how exception messages are 
> generated. They take a string, and that pretty much means that 
> either they're given a string literal (which can be @nogc but 
> does not allow for customizing the error message with stuff 
> like what the bad input was), or they're given a constructed 
> string (usually by using format) - and that can't be @nogc.
>
> And you can't even create an alternate constructor to get 
> around the problem. Everything relies on the msg member which 
> is set by the constructor. Code that wants the message accesses 
> msg directly, and when the exception is printed when it isn't 
> caught, it's msg that is used. Not even overiding toString gets 
> around the issue. For instance, this code
>
> class E : Exception
> {
>     this(int i, string file = __FILE__, size_t line = __LINE__)
>     {
>         super("no message", file, line);
>         _i = i;
>     }
>
>     override string toString()
>     {
>         import std.format;
>         return format("The value was %s", _i);
>     }
>
>     int _i;
> }
>
> void main()
> {
>     throw new E(42);
> }
>
> prints
>
> foo.E at foo.d(20): no message
> [...]
>

---
class E : Exception
{
     this(int i, string file = __FILE__, size_t line = __LINE__)
     {
         super("no message", file, line);
         _i = i;
     }

     override void toString(scope void delegate(in char[]) sink) 
const
     {
         import std.format;
         sink(format("The value was %s", _i));
     }

     int _i;
}

void main()
{
     throw new E(42);
}
---

prints "The value was 42".
Personally, I use a string literal for msg in the constructor, 
add some value members to the exception, and then override the 
above toString.