Re: How do I store an immutable value into a mutable associative array?

[PlatisYialos]

On Tuesday, 6 November 2012 at 18:25:46 UTC, Ali Çehreli wrote:

On 11/06/2012 09:48 AM, PlatisYialos wrote:

On Tuesday, 6 November 2012 at 17:23:41 UTC, PlatisYialos wrote:

Errmm! Here's a better example, but with the same results:

----------------------------
module test;

void noparens() {
immutable char[char] aMap;
char a = 'a';
immutable char b = 'b';
aMap[a] = b;
}

void withparens() {
immutable(char)[char] aMap;
char a = 'a';
immutable char b = 'b';
aMap[a] = b;
}
-----------------------------

I think it is not surprising why noparens() doesn't work: aMap is immutable.

withparens() would be expected to work but again, 'aMap[a] = b' is seen as a mutation of element aMap[a]. If associative arrays had an insert() function, then we would expect it to work.

Here is a quick solution:

/* I used 'inout' to accept mutable, const, and immutable value types */

void insert(K, V)(ref immutable(V)[K] aa, K key, inout V value)
{
    /* I would like to use the following simpler code:
     *
     *     auto mutable = cast(V[K])aa;
     *     mutable[key] = value;
     *

* It did not work when 'aa' was null. The later initialization of

     * 'mutable ' would affect only 'mutable', not 'aa'.
     */
    V[K] * mutable = cast(V[K]*)&aa;
    (*mutable)[key] = value;
}

// ...

  aMap.insert(a, b);

Ali

Thank you, Ali, for going above and beyond on this one!