Encoding a data structure based on TLV encoding

I have to encode the parameters present inside a structure based on X.690 encoding.
Suppose, my structure is:

Code:

struct Data_Struct
   {
      parameter1
      parameter2
      parameter3
   }

Some or all of these parameter may be having valid data, say, parameter1 and parameter3.
Then I am supposed to encode only parameter1 and parameter3 using TLV encoding.
Do I have to follow a sequencial procedure to follow this. Like,

Code:

Check whether parameter 1 is present
    If present, find the tag of the parameter from a lookup table and encode it
Check whether parameter 2 is present
    If present, find the tag of the parameter from a lookup table and encode it
Check whether parameter 3 is present
    If present, find the tag of the parameter from a lookup table and encode it

As the procedure is repetative can I modularize it? What will be the best way to do it? Is there any way to access the parameters sequentially? How can I have a relationship between the parameter and it's tag? Length of the value is variable.

my recommendation is to put your struct(s) that needs to be encoded in its own header file, and then write a program that can parse that header file and generate the encoder/decoder automatically.

I say create a TLV class first and available encoding as an enum with a value equal to the linked tag.

Code:

enum encoding { ASCII = 0x01, binary = 0x02};

class TLV 
{
   unsigned char tag; //a byte
   std::vector<unsigned char> value; //a byte array
public:
   TLV(unsigned int char, int length, const char* s) {
      ....
   }
};

Then create a parser, lets assume that you have a string literal as an input

Code:

void parser(const char* input, std::vector<TLV>& output) {
   ...
}

Finally, you just need to encode depending on the tag. You can do so by

Code:

vector<TLV> tlvs = parser("010231310203112233");
for (int i = 0; i < tlvs.size(); ++i) {
    if (tlvs[i].tag == ASCII) {
       ....
    } else if (tlvs[i].tag == binary) {
       ...
    }
}
//this could potentially output
//parameter_1 = "11"
//parameter_3 = 0x112233