DIP39

DIP 39: Safe rvalue references: compatible with DIP38, backwards compatible, safe against ref/nonref code evolution.

Title:	Safe rvalue references: compatible with DIP38, backwards compatible, safe against ref/nonref code evolution.
DIP:	39
Version:	1
Status:	Draft
Created:	2013-05-10
Last Modified:	2013-05-10
Author:	Timothee Cour
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Abstract

We propose to introduce rvalue references that are:

• safe: guarantees memory safety so that references will always point to valid memory.
• compatible with DIP38: can use same inref/outref internal compiler annotation for input references that can be returned by ref by a function.
• backwards compatible: current valid D code will continue to work without change. In addition, additional code becomes valid with call site rvalue ref annotation.
• safe against ref/nonref code evolution: call site rvalue ref compulsory annotation turns ref/nonref changes into compile errors instead of silently changing code behavior.
• both const ref or ref can be used with rvalue refs (more flexible than C++)
• no call site ref annotation when input ref argument is already an lvalue (different from C#), for backwards compatibility (and making it less verbose)
• compatible with UFCS

Details

Suppose we have a function that takes an input by ref: T2 fun(ref T a); We can use it as before with an lvalue x (backwards compatible): fun(x); We can use it with an rvalue r and a call site annotation indicating to convert the rvalue to an lvalue via a temporary: I propose the yet unused symbol '^' to denote this (note, it is used in some C++ extensions but that's irrelevant), although there are alternatives, see section: 'alternative symbols for call site rvalue annotation'.

fun(r^);

Alternative symbols for call site rvalue annotation

• fun(r^);//proposed postfix annotation, compatible with left-to-right pipelines in D: [1,2].sort.map!fun.uniq
• fun(^r);//prefix annotation is compatible with '&' location wrt argument
• fun(ref r);//reminds of C# call site annotation
• fun(r@);//# has UDA meaning in D, but that could be made unambiguous
• fun(r#); //# has a special line reordering meaning in D, but that could be made unambiguous

Copyright

This document has been placed in the Public Domain.