std.range.interfaces : InputRange moveFront

[Ali Çehreli]

On 12/01/2017 01:11 AM, Johan Engelen wrote:

> I tested it and it works like you wrote, but the behavior is different 
> for an array of integers...:
> 
> auto a = [ 1,2,3 ];
> writeln(a.front); // 1
> auto b = a.moveFront();
> writeln(b); // 1
> writeln(a.length); // still 3
> writeln(a.front); // still 1
> 
> -Johan

Good catch, which affects my struct S example as well.

It's a documentation bug as it does not mention different behavior 
depending on elaborate copy constructor:

   https://github.com/dlang/phobos/blob/master/std/range/primitives.d#L1848

ElementType!R moveFront(R)(R r)
{
     static if (is(typeof(&r.moveFront)))
     {
         return r.moveFront();
     }
     else static if (!hasElaborateCopyConstructor!(ElementType!R))
     {
         return r.front;
     }
     else static if (is(typeof(&(r.front())) == ElementType!R*))
     {
         import std.algorithm.mutation : move;
         return move(r.front);
     }
     else
     {
         static assert(0,
                 "Cannot move front of a range with a postblit and an 
rvalue front.");
     }
}

Ali