Encoding a data structure based on TLV encoding

[Sajas K K]

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

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,

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.

[Elkvis]

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.

[C_ntua]

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

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

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

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

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