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OMG IDL defines unions to be encapsulated discriminated unions: the discriminator itself must be encapsulated within the union.
In addition, the OMG IDL union discriminants must be constant expressions. The discriminator tag must be a previously defined
long, short, unsigned long, unsigned short, char, boolean, or enum constant. The default case can appear at most once in the
definition of a discriminated union, and case labels must match or be automatically castable to the defined type of the discriminator.
The following definition for a discriminated union in OMG IDL
// OMG IDLenum UNION_DISCRIMINATOR
{dChar=0,dShort,dLong,dFloat,dDouble
};
union UNION_OF_CHAR_AND_ARITHMETICswitch(UNION_DISCRIMINATOR){
case dChar: char c;case dShort: short s;case dLong: long l;case dFloat: float f;case dDouble: double d;default: octet v[;8];
};
is mapped into encapsulated unions in Microsoft IDL as follows:
// Microsoft IDL
typedef enum [v1 enum]{dchar=0,dShort,dLong,dFloat,dDouble} UNION_DISCRIMINATOR;
typedef union switch (UNION_DISCRIMINATOR DCE_d){case dChar: char c;case dShort: short s;case dLong: long l;case dFloat: float f;case dDouble: double d;default: byte v[8];}UNION_OF_CHAR_AND_ARITH