Question

I have a byte stream I need parsed into a struct, and I also need to be able to parse the struct back to a byte stream.

Below is an example of what I want where I’ve used BitConverter to parse the values. I hope there is a more efficient way of doing this, because my structs are HUGE!

ref struct TestStruct {     int TestInt;     float TestFloat; };  int main(array<System::String ^> ^args) {     // populating array - just for demo, it's really coming from a file     array<unsigned char>^ arrBytes = gcnew array<unsigned char>(8);     Array::Copy(BitConverter::GetBytes((int)1234), arrBytes, 4);     Array::Copy(BitConverter::GetBytes((float)12.34), 0, arrBytes, 4, 4);      // parsing to struct - I want help     TestStruct^ myStruct = gcnew TestStruct();     myStruct->TestInt = BitConverter::ToInt32(arrBytes, 0);     myStruct->TestFloat = BitConverter::ToSingle(arrBytes, 4);      String^ str = Console::ReadLine();     return 0; } 

Answer 1

For stuff like this, you usually use a code generator. Let’s assume the source looks like this:

struct a {     int i; }  struct b {     string name;     struct a a; } 

What you do is you write a simple parser which searches the source (probably some header file) for ‘struct’, then you read the name of the struct (anything between ‘struct’ and ‘{‘). Write this to the output:

cout << 'struct ' << name << ' * read_struct_' << name << ' (stream in) {' << NL     << '    struct ' << name << ' * result = malloc (sizeof(struct ' << name << '));' NL parseFields (headerStream); cout << '    return result;' << NL << '}' << NL ; }  

Note my C++ is a bit rusty so this probably doesn’t compile but you should get the idea.

In parseFields, you read each line and split it into two parts: Anything before the last space (i.e. ‘int’ in the first example) and the stuff between the last space and and the ‘;’. In this case, that would be ‘i’. You now write to the output:

cout << 'read_' << fieldType << '(in, &result->' << fieldName << ');' << NL; 

Note: You’ll need to replace all the spaces ub the field type with ‘_’.

In the output, this looks like so:

struct a * read_struct_a (stream in) {    struct a * result = malloc(sizeof(struct a));    read_int(in, &result->i);    return result; } 

This allows you to define how to read or write an int somewhere else (in a utility module).

Now, you have code which reads the structure definitions from a header file and creates new code that can read the structure out of some stream. Duplicate this to write the structure to a stream. Compile the generated code and you’re done.

You will also want to write unit tests to verify that the parsing works correctly 🙂 Just create a structure in memory, use the write methods to save it somewhere and read it back again. The two structures should be identical, now. You will want to write a third code generator to create code to compare two structures.