string and char[]

[Steven Schveighoffer]

On Fri, 08 Apr 2011 14:57:52 -0400, spir <denis.spir at gmail.com> wrote:

> On 04/08/2011 03:13 PM, Steven Schveighoffer wrote:
>> On Fri, 08 Apr 2011 06:44:42 -0400, Simen kjaeraas  
>> <simen.kjaras at gmail.com> wrote:
>>
>>> On Fri, 08 Apr 2011 12:46:08 +0200, Morlan <home at valentimex.com> wrote:
>>>
>>>> It is OK if I write
>>>>
>>>> int[char[]] asr;
>>>> asr["hello"] = 10;
>>>>
>>>> but the following does not compile:
>>>>
>>>> char[] car = "hello";
>>>>
>>>> What is the explanation for this behaviour?
>>>
>>> The first should not be allowed. It is a mistake to use non-immutable
>>> keys for an associative array.
>>
>> int[char[]] asr;
>> pragma(msg, typeof(asr).stringof);
>>
>> outputs:
>>
>> AssociativeArray!(const(char)[],int)
>>
>> So the compiler adds const to the keys, which is why it works.
>>
>> Do I think this is the correct behavior? Absolutely not. First, it  
>> prevents
>> nothing as far as modifying keys (const accepts mutable keys as well as  
>> const
>> and mutable ones). Second, I believe you should be able to use whatever  
>> key
>> constancy you want. We should just say if you do the wrong thing, it's
>> undefined. Maybe @safe code can only use immutable keys. Third, if it  
>> must be
>> illegal to have an AA with mutable keys, it should be an error, not  
>> silently
>> change to const.
>
> I agree on points 1 & 3. "Second" looks dangerous to me.

Dangerous, yes.  But immutable objects are typically not easy to deal  
with.  For one, you can't have tail-immutable objects (currently), so  
implementation of such a container is going to be a pain.  In fact,  
dcollections simply doesn't work if you have fully immutable types as keys.

In reality, most times you are not using something as a key and somewhere  
else simultaneously.  So while theoretically dangerous, it's easy to write  
code that isn't dangerous.

-Steve