An xsd:uniontype is used to specify that one of several simple types can be
used for a specific value. This type is mapped to a C structured type that contains an
identifier for the selected type and a union of all of the possible types. Atomic types (i.e.
those that use a single processor storage unit such as integer or Boolean) are stored as
values in the union whereas compound or structured types (such as the structure used to
represent a hexBinary type) are stored as pointers.
The general mapping is as follows:
XSD type:
<xsd:simpleType name="TypeName">
<xsd:union memberTypes="Type1 … TypeN"/>
</xsd:simpleType>
Generated C code:
/* choice tag constants */
#define T_TypeName_type1 1
...
#define T_TypeName_typeN N
typedef struct TypeName {
OSUINT16 t;
union {
/* t = 1 */
Type1 type1;
...
/* t = N */
TypeN typeN;
} u;
} TypeName;
Generated C++ code:
class TypeName : public OSRTBaseType {
public:
// tag constants
enum {
T_type1 = 1,
...
T_typeN = N,
} ;
OSUINT16 t;
union {
/* t = 1 */
Type1 type1;
...
/* t = N */
TypeN typeN;
} u;
...
} ;
Notes:
Where typename begins with a lowercase letter above (for example, Type1is shown as type1in places), it means the actual typename is used with the first letter set to lowercase.
The choice tag constants ( T_TypeName_type) are the identifiers of each of the particular values in the union. The selected value is stored in the t member variable of the generated structure. In the case of C++, the tag values are in the form of an enumconstruct within the class containing enumerations of the form T_type.
The member variables in the union may be stored as values (if atomic) or as pointers to a value of the item (if structured).
The generated C++ class contains additional methods to get, set, or query the union value. These are in the form of get_type, set_type, and is_typerespectively.