Why does toString() exist? It seems useless.

[Sean Kelly]

Lars Ivar Igesund wrote:
> 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.

Oops, I stand corrected.


Sean