[Issue 5097] New: Safer unions with @tagged

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5097

           Summary: Safer unions with @tagged
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-10-21 18:34:53 PDT ---
D language is designed to be safer than C and C++, yet it's not Java, it has to
offer less safe features too, like unions.

The failure of the Cyclone language shows that the safety features need to be
light, minimal, not syntactically intrusive.

The simpler way to make unions safe is to introduce an attribute as @tagged.

When an union has a @tagged attribute, one word is added by the compiler to the
union, that makes sure (with runtime tests) that the last written field is
read. In release mode this invisible field and their tests vanish (a better
feature is to test and enforce this at compile time, but this requires a more
complex type system that D doesn't have).

(If you want even more safety, unions with the @tagged attribute may not allow
to take the address of fields, to avoid bypassing the runtime cheeks.)

In the end the programmer that wants to use safer unions in D may just add the
@tagged attribute, nothing else is required, and the union semantics is
unchanged (just its size is changed in non-release mode). So this is a very
light and easy to use safety net:


@tagged union Foo {
    int i;
    float f;
}

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