enum data type

Enumeration Data Type

IDL supports enumerated types with syntax identical to that of their C counterpart, as in the following example:

enum MYCOLOR {MYRED, MYGREEN, MYBLUE}; 

The enumeration can be used as a parameter to a method, as in the following example:

// Interface method definition HRESULT GetEnum([out] enum MYCOLOR* pVal); // Server code STDMETHODIMP CMyExplore::GetEnum(enum MYCOLOR *pVal) {  *pVal = MYRED;   return S_OK; } // Client code enum MYCOLOR color; HRESULT hr = pMyExplore->GetEnum(&color); 

To simplify enum data type declaration, enum definitions can also be used along with C-style typedefs. In this case, we can drop the keyword enum from the interface method declaration, as shown below:

typedef enum { MYRED, MYBLUE, MYGREEN } MYCOLOR; // Interface method definition HRESULT GetEnum([out] MYCOLOR* pVal); // Server code STDMETHODIMP CMyExplore::GetEnum(MYCOLOR *pVal) {  *pVal = MYRED;   return S_OK; } // Client code MYCOLOR color; HRESULT hr = pMyExplore->GetEnum(&color); 

Note that an enum type variable can be assigned only one value from the possible list of values. It cannot be used to pass a combination of values, as in the following case:

// Server code STDMETHODIMP CMyExplore::GetEnum(MYCOLOR *pVal) {  *pVal = MYRED | MYGREEN;   return S_OK; } 

If the value to be returned does not exactly match one of the possible values, the marshaler fails to marshal the value.

If a method intends to pass a combination of enumerated values as a parameter, declare the parameter as a long type, instead of the enum type.

An enum can be uniquely identified by a GUID, if desired, as shown here:

typedef [  uuid(2B930581-0C8D-11D3-9B66-0080C8E11F14), ] enum {MYRED, MYGREEN, MYBLUE } MYCOLOR; 

By default, the NDR format for enum is a 16-bit unsigned short. To speed up transmission on 32-bit architectures, it is desirable to have enum values transmitted as 32 bits. IDL defines a v1_enum attribute just for this case:

typedef [  v1_enum,   uuid(2B930581-0C8D-11D3-9B66-0080C8E11F14), ] enum {MYRED, MYGREEN, MYBLUE } MYCOLOR; 

The enumeration and each enumeration constant can have a "helpstring" as shown here:

typedef [  v1_enum,   uuid(2B930581-0C8D-11D3-9B66-0080C8E11F14),   helpstring("This is my color enumeration") ] enum {  [helpstring("This is my red")] MYRED,   [helpstring("This is my green")] MYGREEN,   [helpstring("This is my blue")] MYBLUE }MYCOLOR; 

The default value for an enumeration starts from zero. If desired, each enumeration item can be assigned an individual value, as shown here:

typedef [  v1_enum,   uuid(2B930581-0C8D-11D3-9B66-0080C8E11F14),   helpstring("This is my color enumeration") ] enum {  [helpstring("This is my red")] MYRED  = 0x0001,   [helpstring("This is my green")] MYGREEN = 0x0002,   [helpstring("This is my blue")] MYBLUE = 0x0004 }MYCOLOR;