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Putting the Code Together
Listing 7 illustrates the parser class.
Listing 7 Base Parser Class
class ParseAnyDataType {
public:
virtual ~ParseAnyDataType() {};
virtual void ParseData() {};
protected:
ParseAnyDataType() {};
};
Listing 8 illustrates the subclass of ParseAnyDataType.
Listing 8 Main ASN.1 Parser Class
class ParseDataStream : ParseAnyDataType {
public:
explicit ParseDataStream(const char * dataSet);
~ParseDataStream() { }
void parseAsnElement(const char * dataSet, int * offset,
int elementLength, int overallLength); // Recursive member function
void echoDataStream(const char * dataSet, int offset);
};
The constructor for the class ParseDataStream is illustrated in Listing 9.
Listing 9 Main Parser Constructor
ParseDataStream::ParseDataStream(const char * dataSet) {
int offset = 0;
printf("Making the first call to parseAsnElement()\n");
parseAsnElement(dataSet, &offset, dataSet[1], strlen(dataSet));
}
The code in Listing 9 starts the recursive processing of the ASN.1 data. The parseAsnElement() member function is called by each of the individual parsers (Listings 3–6). I mentioned at the beginning that using recursion can help produce elegant solutions. Recursion has an interesting characteristic that I’ve never seen discussed: it is a type of reuse. In effect, when you call a method recursively—as in the case of parseAsnElement()—you are reusing the code, which can facilitate a certain economy in the solution. Take a look at Listing 10 to see the complete source code for the parseAsnElement()method.
Listing 10 Recursive Member Function—parseAsnElement()
void ParseDataStream::parseAsnElement(const char * dataSet,
int * offset,
int elementLength,
int overallLength) {
char dataSetType;
int i;
while (*offset < overallLength - 1)
{
dataSetType = dataSet[*offset];
elementLength = dataSet[*offset + 1];
switch (dataSetType)
{
case SEQ_CLASS:
*offset += 2;
parseAsnElement(dataSet, offset, elementLength, overallLength);
break;
case INT_CLASS:
for (i = *offset + 2; i <= *offset + elementLength + 1; i++)
printf("%x", dataSet[i]);
printf("\n");
*offset += 4;
parseAsnElement(dataSet, offset, elementLength, overallLength);
break;
case STR_CLASS:
printf("String character ");
for (i = *offset + 2; i <= *offset + elementLength + 1; i++)
printf("%c", dataSet[i]);
printf("\n\n");
*offset += elementLength + 1;
parseAsnElement(dataSet, offset, elementLength, overallLength);
break;
default:
printf("Unknown type\n");
break;
}
}
}
I think Listing 10 achieves a lot for such a small amount of code! (Please see the "Additional Reading" section for the complete source code.) If you want to execute the code, just drop the two files into a Microsoft Visual C++ console project, build and run.