On Tuesday, 9 June 2015 at 17:38:00 UTC, Steven Schveighoffer
wrote:
But passing a large rvalue by value does not involve any moving
of data, and can be abstracted to a pass by ref if needed. It's
never less performing than an explicit pass by ref.
Thanks Steve.
The problem I see here on Win64 is that the current `auto ref`
implementation for templates, i.e., 2^N function instantiations
for N lvalue/rvalue combinations, seems totally useless for value
types for which the ABI enforces pass-by-ref anyway.
In essence, `auto ref T` for templated functions on Win64 boils
down to a simple `ref` accepting rvalue arguments iff T cannot be
passed by value. All what's needed is constructing the rvalue on
the caller's stack and destroying it after the call; the callee
isn't affected at all, so no need for 2^N function versions, 1 is
enough. Note that I haven't inspected the DMD code yet, but I
assume it makes no exception for Win64 in that regard.
No opinions on the semantics for `in` I proposed earlier? `in T`
would be something like a non-escapable `const auto ref T`
accepting rvalues without codebloat and avoiding the indirection
for small POD types T. Wouldn't that eliminate 99% of the use
cases for `auto ref`, improve readability substantially and be
the solution for all the C++ guys missing the rvalue-bindability
of `const T&`?
