Open Type

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Note: The X.680 Open Type replaces the X.208 ANY or ANY DEFINED BY constructs. An ANY or ANY DEFINED BY encountered within an ASN.1 module will result in the generation of code corresponding to the Open Type described below.

An Open Type as defined in the X.680 standard is specified as a reference to a Type Field in an Information Object Class. The most common form of this is when the Type field in the built-in TYPE-IDENTIFIER class is referenced as follows:

	TYPE-IDENTIFIER.&Type
 

See the section in this document on Information Objects for a more detailed explanation.

The Open Type is converted into a C or C++ structure used to model a dynamic OCTET STRING type. This structure contains a pointer and length field. The pointer is assumed to point at a string of previously encoded ASN.1 data. When a message containing an open type is decoded, the address of the open type contents field is stored in the pointer field and the length of the component is stored in the length field.

The general mapping of an Open Type to C/C++ is as follows:

    ASN.1 production:    <name> ::= ANY
 

 
    Generated C code:    typedef ASN1OpenType <name>;
 

    Generated C++ code:  typedef ASN1TOpenType <name>;
 

The difference between the two types is the C++ version contains constructors to initialize the value to zero or to a given open type value.

If the -tables command line option is selected and the ASN.1 type definition references a table constraint, the code generated is different. In this case, ASN1OpenType above is replaced with ASN1Object (or ASN1TObject for C++). This is defined in asn1type.h as follows:

typedef struct {  /* generic table constraint value holder */
 
   ASN1OpenType encoded;
 
   void*        decoded;                                    
 
   OSINT32      index;      /* table index */
 
} ASN1Object;
 

This allows a value of any ASN.1 type to be represented in both encoded and decoded forms. Encoded form is the open type form shown above. It is simply a pointer to a byte buffer and a count of the number of byes in the encoded message component. The decoded form is pointer to a variable of a specific type. The pointer is void because there could be a potentially large number of different types that can be represented in the table constraint used to constrain a type field to a given set of values. The index member of the type is for internal use by table constraint processing functions to keep track of which row in a table is being referenced.