bug with std.range.zip? range with opEquals(const) const not allowed inside zip

[Jonathan M Davis]

On Sunday, August 26, 2012 14:47:38 Jens Mueller wrote:
> What is the ref situation?
> I thought ref is used as an optimization that allows passing lvalues
> more efficiently whereas without ref is needed for rvalues.

ref doesn't necessarily have anything to do with optimizations. It makes it so 
that any operations done on the function parameter inside the function are 
also done on the variable passed in. The parameter is essentially an alias for 
the variable passed in.

You can choose to use ref in order to avoid a copy, but it's also arguably bad 
practice to have a function take an argument by ref if it's not actually going 
to mutate the original, since that's confusing.

_const_ ref is more for avoiding the copy, since it's like ref except that you 
can't actually mutate the variable. So, it makes sense to use const ref 
specifically to avoid unnecessary copies.

However, while in C++, you can pass rvalues to const&, in D, you can't. It 
doesn't matter if the ref is const or not, it still requires an lvalue. This 
solves certain issues caused by C++'s choice (issues primarily related to 
function overloading IIRC), but it makes it rather difficult to declare a single 
function which takes either an lvalue or an rvalue but which tries to avoid 
unnecessary copying.

D's auto ref essentially does it (rvalues are then always passed by rvalue and 
lvalues are always passed by lvalue), but it only works with templates, 
because it has to create two versions of the function.

So, what we'd _like_ to have is a solution where you can mark a parameter in a 
way similar to auto ref or C++s const& and have the function not care whether 
it's passed an lvalue or rvalue and have it avoid copying as much as possible 
and have it work with _all_ function types, not just templated ones, but we 
haven't sorted out how to do that yet. _That's_ the ref situation that needs 
to be solved.

- Jonathan M Davis