Re: using yacc union in managed c++

George Neuner <gneuner2@comcast.net>
13 Apr 2007 01:35:58 -0400

          From comp.compilers

Related articles
using yacc union in managed c++ rastrahm@yahoo.com (ras) (2007-04-11)
Re: using yacc union in managed c++ cdodd@acm.org (Chris Dodd) (2007-04-13)
Re: using yacc union in managed c++ gneuner2@comcast.net (George Neuner) (2007-04-13)
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From: George Neuner <gneuner2@comcast.net>
Newsgroups: comp.compilers
Date: 13 Apr 2007 01:35:58 -0400
Organization: Compilers Central
References: 07-04-032
Keywords: yacc, C++

On 11 Apr 2007 23:34:10 -0400, "ras" <rastrahm@yahoo.com> wrote:


>Is there a way to implement the union in a managed c++ yacc parser?


This is a solution from MSDN magazine:


The common language runtime (CLR) doesn't understand unions, but you
can use an ordinary __value struct with some special voodoo to tell it
where your members go. The magic attributes are StructLayout and
FieldOffset. In managed C++, it looks like this:


[StructLayout(LayoutKind::Explicit)]
public __value struct MyUnion {
    [FieldOffset(0)] int i;
    [FieldOffset(0)] double d;
};


This tells the CLR that the integer i and the double d are both at
offset zero (that is, the first items) in your struct, which makes
them overlap and, in effect, the struct is a union. You can then use
MyUnion in your __gc class, like so:


public __gc class CUnionClass {
public:
    // can access this directly because it's public
    MyUnion uval;
};


With CUnionClass defined, you can access the members i and d directly
through uval from any .NET-compliant language. In C# it looks like the
following code snippet:


CUnionClass obj = new CUnionClass();
obj.uval.i = 17;
obj.uval.d = 3.14159




See http://msdn.microsoft.com/msdnmag/issues/04/08/CQA/ for more.


Hope this helps.
George



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