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digitalmars.D.bugs - [Issue 7014] New: Better union initialization syntax

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7014

           Summary: Better union initialization syntax
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



After using several unions for a while in D, I think the syntax to initialize
fields of unions is not handy enough. An idea from GNU-C:


import std.stdio;
union Foo { int i; double d; };
void main() {
    int x;
    double y;
    Foo u = void;
    auto u1 = cast(Foo)x;
    writeln(typeof(u1).stringof);
    auto u2 = cast(Foo)y; // Error: e2ir: cannot cast y of type double to type
Foo
    writeln(typeof(u2).stringof);
}


See:
http://gcc.gnu.org/onlinedocs/gcc/Cast-to-Union.html#Cast-to-Union


But I don't like that GNU-C idea, a problem with cast() is that it's not
precise enough, if there are two fields like this, what is a good way to assign
's' or 'u' using a cast()?

union Foo { int s; uint u; };

So I'd like a more explicit syntax.


A solution using named arguments seems good:

union Foo { int s; uint u; };
void bar(Foo f) {}
void main() {
    bar(Foo(u:5));
}

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Nov 25 2011
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7014


timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |timon.gehr gmx.ch





 
 But I don't like that GNU-C idea, a problem with cast() is that it's not
 precise enough, if there are two fields like this, what is a good way to assign
 's' or 'u' using a cast()?
 
 union Foo { int s; uint u; };


cast(Foo)cast(uint)exp
cast(Foo)cast(int)exp

You understand that both are assigned, regardless of which one you choose and
ergo it is never a problem?

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Nov 26 2011
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7014


Denis Shelomovskij <verylonglogin.reg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |verylonglogin.reg gmail.com



18:30:34 MSD ---
Another option may be to extend "static initialization" syntax:
---
struct S { int i; double d; }
union U { int i; double d; }
void f(T)(T){}

void main()
{
    S s = { d: 5 }; // OK
    U u = { d: 5 }; // NG because of Issue 7727

    // Original suggestion:
    f(S( d: 5 )); // yes, let's not separate structs and unions
    f(U( d: 5 ));

    // Another option:
    f(cast(S) { d: 5 });
    f(cast(U) { d: 5 });
}
---

I'd like to require a `cast` even if the argument type is known.

And about original suggestion:
Having naming arguments in D (not only in this case) looks like a good idea
allowing one another coding style.

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Sep 30 2013