What is the preferred method for testing the existence of a key in an associative array?

[simendsjo]

On Saturday, 14 September 2013 at 10:50:17 UTC, Jacob Carlborg 
wrote:
> On 2013-09-13 16:17, Orvid King wrote:
>
>> Well, I usually do it as:
>>
>> int[string] someCache;
>>
>> int getValue(string key)
>> {
>>     if (auto val = key in someCache)
>>         return *val;
>>     return someCache[key] = -3;
>> }
>
> That doesn't work with generic code. I mean, -3 can be a legal 
> value. There are many types that doesn't have an invalid value, 
> like pointers do.

I don't understand. What doesn't work? If the key exists, val !is 
null, if it doesn't, val is null.