The ASN1 specification defines the SEQUENCE, SEQUENCE OF, SET and SET OF keywords to denote sequences and sets of data. Sequences mean ordered lists and sets are unordered. In essance, a SEQUENCE is a structure of data, and a SEQUENCE OF is an list of data. The SET and SET OF keywords are the unordered counterparts to the SEQUENCE keywords.

Boost.ASN1 has a set of templates used for dealing with PDU objects. The base templates are called, pdu_base_t and pdu_base_of_t. From these two base objects are derived the templates that deal with SEQUENCE and SET keywords. These templates are named sequence<>, sequence_of<>, set<> and set_of<>. These templates are fairly easy to use; you create a structure with the variables you require in the PDU and then use that structure as the type parameter for one of the templates. For instance:

Authenticate := SEQUENCE {
  MessageID INTEGER,
  UserName IA5STRING,
  Password OCTET STRING
  }

struct authenticate
{
  integer_t MessageID;
  ia5string_t UserName;
  octectstring_t Password;
};
sequence<authenticate> Authenticate;

The sequence<> template allows for the user to directly access the structure that is contained as the typename, for example:

Authenticate.MessageID = rng();

The sequence_of template is simply a vector of ASN1 objects, so, you would use it like an array:

sequence_of<ia5string_t> string_list;
string_list[0] = "First";
string_list[1] = "Second";
string_list[3] = "Third";

This type conversion funcionality is only the first step to encoding or decoding your data. You must give these functions an asn_base_ptr to either convert from or to.

asn_base_ptr pduPtr;
string_list.to(pduPtr);

This now gives you a PDU pointer that the asn_encode and asn_decode can use to encode/decode the actual ASN1 packet.