Why does toString() exist? It seems useless.

[Lars Ivar Igesund]

Sean Kelly wrote:

> Don wrote:
>> Robert Fraser wrote:
>>> Benji Smith wrote:
>>>> Don wrote:
>>>>> I cannot understand the rationale for a toString() member function
>>>>> which doesn't support formatting.
>>>>
>>>> I agree.
>>>>
>>>> In Java, the toString() method *must* exist on the base Object class,
>>>> because of constructs like this:
>>>>
>>>>    String s = "hello" + (new World());
>>>>
>>>> Without an implementation of toString(), it'd be impossible to
>>>> support those kinds of automatic String conversion and concatenation
>>>> (which actually do end up being pretty handy in logging & debugging
>>>> statements).
>>>>
>>>> But since D doesn't support implicit String conversion, it seems
>>>> pretty pointless.
>>>
>>> I think both Tango and phobos allow objects to be converted to strings
>>> when specified as parameters to a formatting function -- i.e.
>>> Stdout.format("{0}", obj) will call obj.toString().
>> 
>> And that's the problem -- it's a really poor design, totally unsuitable
>> for that purpose.
>> 
>> Simple challenge:
>> 
>> class Foo {
>> public:
>>  double x;
>> }
>> 
>> 1. You want it to output x in the default way for the current locale.
>> (eg, 5,35 or 5.35 depending on which country you're in).
>> Write toString().
>> 2. Now write toString() such that:
>> 
>> double y = 3.156892123;
>> Foo z = new Foo;
>> z.x = 3.156892123;
>> writefln("%.3f, %.3f", y, z);
>> writefln("%.4f, %.4f", y, z);
>> 
>> (or Tango equivalent) produces the same results for y and z.
>> 
>> Both of these should be trivial.
> 
> We had actually asked for some compiler changes to support this sort of
> formatting (necessary since the function signature for toString is
> hardcoded in the compiler for structs), but the request was denied,
> presumably as unnecessary.  I don't remember the details of the request
> however--Lars would have to provide that.

It was more of a suggestion that something accommodating this should exist,
with toString(char[] fmt) as an initial suggestion. I think it just slipped
away as it wasn't a good enough solution, not really denied.

-- 
Lars Ivar Igesund
blog at http://larsivi.net
DSource, #d.tango & #D: larsivi
Dancing the Tango