Unions

Some features of the C language, such as unions, require special MIDL keywords to support their use in remote procedure calls.

A union in the C language is a variable that holds objects of different types and sizes. The developer usually creates a variable to keep track of the types stored in the union. To operate correctly in a distributed environment, the variable that indicates the type of the union, or the "discriminant," must also be available to the remote computer. MIDL provides the switch_type and switch_is keywords to identify the discriminant type and name.

MIDL requires that the discriminant be transmitted with the union in one of two ways:

• The union and the discriminant must be provided as parameters.
• The union and the discriminant must be packaged in a structure.

Two fundamental types of discriminated unions are provided by MIDL: non-encapsulated_union and encapsulated_union. The discriminant of a nonencapsulated union is another parameter if the union is a parameter. It is another field if the union is a field of a structure. The definition of an encapsulated union is turned into a structure definition whose first field is the discriminant and whose second and last fields are the union.

The following example demonstrates how to provide the union and discriminant as parameters:

typedef [switch_type(short)] union {
    [case(0)]    short     sVal;
    [case(1)]    float     fVal;
    [case(2)]    char      chVal;
    [default]    ;
} DISCRIM_UNION_PARAM_TYPE;
 
short UnionParamProc(
    [in, switch_is(sUtype)] DISCRIM_UNION_PARAM_TYPE Union,
    [in]                    short                    sUtype);
 

The union in the preceding example can contain a single value: either short, float, or char. The type definition for the union includes the MIDL switch_type attribute which specifies the type of the discriminant. Here, [switch_type(short)] specifies that the discriminant is of type short. The switch must be an integer type.

If the union is a member of a structure, then the discriminant must be a member of the same structure. If the union is a parameter, then the discriminant must be another parameter. The prototype for the function UnionParamProc shows the discriminant sUtype as the last parameter of the call. (The discriminant can appear in any position in the call.) The type of the parameter specified in the switch_is attribute must match the type specified in the switch_type attribute.

The following example demonstrates the use of a single structure that packages the discriminant with the union:

typedef struct {
    short utype;  /* discriminant can precede or follow union */
    [switch_is(utype)] union {
       [case(0)]   short     sVal;
       [case(1)]   float     fVal;
       [case(2)]   char      chVal;
       [default]   ;
    } u;
} DISCRIM_UNION_STRUCT_TYPE;
 
short UnionStructProc(
    [in] DISCRIM_UNION_STRUCT_TYPE u1);
 

The Microsoft RPC MIDL compiler allows union declarations outside of typedef constructs. This feature is an extension to DCE IDL. For more information, see union.