DIP38

DIP34: Static array literals (STILL EDITING -- don't read yet)

Title:	Safe references and rvalue references without runtime checks
DIP:	38
Version:	1
Status:	Draft
Created:	2013-05-06
Last Modified:	2013-05-06
Author:	Timothee Cour
Links:

Abstract

Safe references and rvalue references without runtime checks.

   auto x=[1,2,3]S; 
   static assert(is(typeof(x)==int[3]));

The particular choice of 'S' can be discussed.

Description

Dconf13 introduced safe references enabled by a runtime check (see email thread from Walter: 'Rvalue references - The resolution'). I propose a formulation that is safe, yet doesn't require any runtime check. The compiler automatically annotates any return by ref function with a dependency on input arguments:

eg:

struct U{T x;}
ref T foo(ref T a, ref T b, ref U c, T d){
  if(condition)
    return a;
  else if(condition(b))
    return c.x;
  else
     return d;
}

will be rewritten internally by the compiler as having the signature:

ref(0,2) T foo(ref T a, ref T b, ref T c, T d);

indicating that ref depends on ref arguments 0(a) and 2 (c) (dependency on c is via field access). The other arguments are not in the list because they're not input ref arguments, or not returned.

Second example: when the function is a member (say of a struct), the 'this' parameter is implicitly argument number 0, but the same rules apply:

struct S { T t; ref T fooc(ref T a) { if(condition) return t; else return a;} }

will be rewritten internally by the compiler as having the signature:

struct S { T t; ref(0,1) T fooc(ref T a); }

because there's a ref dependency on 0(this) and 1(a).

Note, if there's no dependency on input ref arguments (for example, it returns by ref a global), then the ref list is empty, ie ref().

Given those compiler generated annotations, it is easy to validate/invalidate ref safety: the rule is: in any return statement of a ref-return function, the ref dependency list can only refer to: A) global variables (gc-alloced, static etc) B) ref inputs This excludes locals.

Examples: taken from Walter's above mentioned email:

//Case A:
    ref T fooa(ref T t) { return t; }
    //=> ref(0) T fooa(ref T t);
    ref T bar() { T t; return fooa(t); } // error since 0 refers to t which is a local

//Case B:
    ref T foob(ref U u) { return u.t; } 
//=>ref(0) T foob(ref U u) { return u.t; } 
    ref U bar() { T t; return foob(t); } // error since 0 refers to t which is a local

//Case C:
    struct S { T t; ref T fooc() { return t; } }
//=>struct S { T t; ref(0) T fooc(); } //0 refers to 'this'
    ref T bar() { S s; return s.fooc(); } // error since 0 refers to this = s, which is local

//Case D:
    Returning ref to uplevel local:

    ref T food() {
        T t;
        ref T bar() { return t; }
//=>ref(0) T bar(); // 0 here indicates an implicit 'this' pointing to local stack allocated variables (wouldn't be the case if t were static variable)
        return bar(); //error since 0 refers to local stack
    }

//case E:
    Transitively calling other functions:

    ref T fooe(T t) { return ref  fooa(t); } //error since we have the compiler rewrite 'ref(0) T fooa(ref T t);' and 0 refers to t, a local variable (t is not a ref variable in fooe).

Final point: the di files will have to write those annotations written down, which shall be done automatically.

Usage

This section has been adapted for MediaWiki.

To start a new DIP you can go to Edit link and copy the source of this DIP, then go to DIP index and add a new item in the list. The DIP number should be one more than the last DIP in the index (for example, if the DIP1 is the last DIP, your DIP should be DIP2). The link in the index should have the form: [[DIPx]], Title, Status, resume. Where x is the DIP number, title is the DIP title and resume is a short description about the DIP.

Save the DIP index page and click on the new red link. Now you are editing the new DIP you just created, now paste the copied source text from this template and replace all the you need.

Remember to update the metadata at the start of the DIP, and keep it as a Draft at the beginning. When your DIP is done, you should announce it in the News Group for discussion, with a subject like this: new DIPx: title (where one more time x is the DIP number and title is the DIP title).

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Recommendations

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Abstract

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Rationale

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NG Announcement

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Copyright

This document has been placed in the Public Domain.