Templates: Array slices not recognized

[Chris via Digitalmars-d-learn]

On Saturday, 18 April 2015 at 16:26:57 UTC, Max Klyga wrote:
> On 2015-04-18 13:46:19 +0000, Chris said:
>
>> The following:
>> 
>> import std.stdio : writefln;
>> import std.range.primitives : isInputRange, hasLength;
>> 
>> void main() {
>>   size_t[] a = [1, 2, 3, 4, 5, 6, 7, 8, 9];
>>   doSomething(a);  // works
>> 
>>   doSomething(a[0..5]);
>> 
>> // ---> Error: template slices.doSomething cannot deduce 
>> function from argument types !()(ulong[]), candidates are:
>>   // slices.d(11): slices.doSomething(R)(ref R r) if 
>> (isInputRange!R && hasLength!R)
>> 
>>   doSomething!(size_t[])(a[0..5]);
>>   // ---> Error: doSomething (ref ulong[] r) is not callable 
>> using argument types (ulong[])
>> }
>> 
>> void doSomething(R)(ref R r)
>>                 if (isInputRange!R && hasLength!R)  // etc..
>> {
>>   foreach (ref n; r) {
>>     writefln("%d * 2 = %d", n, n * 2);
>>   }
>> }

>> 
>> //EOF
>> 
>> a[0..5] is not recognized as size_t[]. If I give the compiler 
>> a hint with !(size_t[]), it complains again, i.e. I can not 
>> pass the slice as a reference.
>> 
>> A workaround is
>> 
>> size_t[] b = a[0..5];
>> doSomething(b);
>> 
>> However, this comes with a serious performance penalty in for 
>> loops (even if I predefine b and reuse it in the loop). 
>> to!(size_t[])(a[0..5]) is even worse.
>> 
>> Any thoughts or tips?
>
> a[0..5] is an R-value, and cannot be passed by reference.
> As you noticed, once you use a variable - everything works 
> because only L-values can be passed by reference.
>
> Also why are you passing slices by reference? Slices do not 
> copy the memory they point to when passed by value.

Doh! You're right! My bad. However, this makes the function less 
generic, but it doesn't matter here.