Re: Anonymous union rules against constructors & assignment

"Greg Herlihy" <>
Sat, 3 Jun 2006 11:08:16 CST
Carl Barron wrote:

Greg Herlihy <> wrote:

And if a union is to be used, then the only approach is to eliminate
the non POD types. I would imagine that a union like the one below
could suffice:

    int main()
        union // anon_1
            char data[4];

            struct // anon_2
                char d0;
                char d1;
                char d2;
                char d3;
        } x, y;

        x.d1 = 23; // Works fine
        x.d2 = 123; // Fine
        y.d1 = 5; // Fine
        y.d2 = x.d1; // Fine

        x.d2 = y.d2; // Fine

 as long as data is not used for array access of the struct. There is
no guarantee that sizeof(anon_2) == 4 or that offsetof(anon_2,d1)==1,
the only guarntess are that d0 and data[0] have the same address and
that sizeof(anon_1)>= max(4,sizeof(anon_2)). Any more assumptions is
non standard, and probably compiler option dependent as well...

With a slightly more elaborate declaration the union can guarantee that
each field in the struct (d1, d2, d3, d4) lines up exactly with the
corresponding element in the array:

    #include <tr1/type_traits>

    typedef std::tr1::aligned_storage<1, 1>::type Aligner;

    union Element
        Aligner u;
        char c;

        Element& operator=(char ch)
            c = ch;
            return *this;

    union U
        Element data[4];

            Element d1;
            Element d2;
            Element d3;
            Element d4;
    } x, y;


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