Question

I am trying to build and write a binary request and have a “is this possible” type question. It might be important for me to mention the recipiant of the request is not aware of the data structure I have included below, it’s just expecting a sequence of bytes, but using a struct seemed like a handy way to prepare the pieces of the request, then write them easily.

Writing the header and footer is fine as they are fixed size but I’m running into problems with the struct “Details”, because of the vector. For now Im writing to a file so I can check the request is to spec, but the intention is to write to a PLC using boost asio serial port eventually

I can use syntax like so to write a struct, but that writes pointer addresses rather than values when it gets to the vector

myFile.write((char*) &myDataRequest, drSize);

I can use this sytax to write a vector by itself, but I must include the indexer at 0 to write the values

myFile.write((char*) &myVector[0], vectorSize);

Is there an elegant way to binary write a struct containing a vector (or other suitable collection), doing it in one go? Say for example if I declared the vector differently, or am I resigned to making multiple writes for the content inside the struct. If I replace the vector with an array I can send the struct in one go (without needing to include any indexer) but I dont know the required size until run time so I don’t think it is suitable.

My Struct

    struct Header
    {   ...     };

    struct Details
    {
           std::vector<DataRequest> DRList;
    };

    struct DataRequest
    {
           short numAddresses;          // Number of operands to be read   Bytes 0-1
           unsigned char operandType;   //                                  Byte 2
           unsigned char Reserved1;     //Should be 0xFF                    Byte 3
           std::vector<short> addressList;  // either, starting address (for sequence), or a list of addresses (for non-sequential) 
    };

    struct Footer
    {   ...     };

Answer 1

It’s not possible because the std::vector object doesn’t actually contain an array but rather a pointer to a block of memory. However, I’m tempted to claim that being able to write a raw struct like that is not desireable:

1. I believe that by treating a struct as a block of memory you may end up sending padding bytes, I don’t think this is desireable.

2. Depending on what you write to you may find that writes are buffered anyway, so multiple write calls aren’t actually less efficient.

3. Chances are that you want to do something with the fields being sent over. In particular, with the numeric values you send. This requires enforcing a byte order which both sides of the transmission agree on. In order to be portable, you should exlicitely convert the byte order to make sure that your software is portable (if this is required).

To make a long story short: I suspect writing out each field one by one is not less efficient, it also is more correct.