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.|
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
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 LV (backwards compatible):
We can use it with an rvalue RV 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'.
Alternative symbols for call site rvalue annotation
2 things to decide on : prefix or postfix annotation, and which annotation to use:
prefix vs postfix:
- postfix fun(RV^): (proposed): compatible with left-to-right pipelines in D: [1,2].sort.map!fun.uniq
- prefix fun(^RV): compatible with '&' location wrt RV argument
This can affect ease of disambiguation wrt existing symbols.
which annotation to use (regardless of prefix/postfix):
- fun(RV^);//unused in D, reminds of a C++ special reference extension
- fun(ref RV);//reminds of C# call site annotation, and reminds of function signature
- fun(RV@);//@ has UDA meaning in D, but that could be made unambiguous
- fun(RV#); //# has a special line reordering meaning in D, but that could be made unambiguous
- fun(RV?); //? has a special (a?b:c) meaning in D, but that could be made unambiguous
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