Conditional compilation on availability?

[Bruno Medeiros]

Kirk McDonald wrote:
> Fredrik Olsson wrote:
>> Might be an easy one, or a missing feature :).
>>
>> But how do I do a conditional compilation on the availability of a 
>> function, variable, type or whatever?
>>
>> For example:
>>
>> class foo(T) {
>>  T[] bar;
>>   static if ("predicate for toString(T) exists") {
>>     char[] allBarsConcatinatedAsString() {
>>       char[] r = ""
>>       foreach (v; bar) {
>>     r ~= toString(v);
>>       }
>>     }
>>   }
>> }
>>
>> What would I type instead of "predicate for toString(T) exists"?
>>
>> // Fredrik
> 
> class foo(T) {
>     static if (is(typeof(toString(T.init)))) {
>         // ...
>     }
> }
> 
> The IsExpression tests whether a given type is valid. typeof gets the 
> type of a given expression. Thus, you can use one inside the other to 
> test whether a given expression exists. Additionally, the expression 
> inside of typeof is not actually executed, so it's safe to "call" the 
> function like this.
> 

Note that that doesn't work if T is a static array, since
typeof(T) != typeof(T.init) but instead typeof(T.init) == typeof(*T) 
(which is viewed by some of us as a nasty inconsistency).
Best to do as in Tom's example.


-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D