从byte []到struct的类型转换

Question

I'm currently working on a small C++ project where I use a client-server model someone else built. Data gets sent over the network and in my opinion it's in the wrong order. However, that's not something I can change.

Example data stream (simplified):

0x20 0x00 (C++: short with value 32)
0x10 0x35 (C++: short with value 13584)
0x61 0x62 0x63 0x00 (char*: abc)
0x01 (bool: true)
0x00 (bool: false)

I can represent this specific stream as :

struct test {
    short sh1;
    short sh2;
    char abc[4];
    bool bool1;
    bool bool2;
}

And I can typecast it with test *t = (test*)stream; However, the char* has a variable length. It is, however, always null terminated.

I understand that there's no way of actually casting the stream to a struct, but I was wondering whether there would be a better way than struct test() { test(char* data) { ... }} (convert it via the constructor)

Answer 1

This is called Marshalling or serialization .

What you must do is read the stream one byte at a time (or put all in a buffer and read from that), and as soon as you have enough data for a member in the structure you fill it in.

When it comes to the string, you simply read until you hit the terminating zero, and then allocate memory and copy the string to that buffer and assign it to a pointer in the struct.

Reading strings this way is simplest and most effective if you have of the message in a buffer already, because then you don't need a temporary buffer for the string.

Remember though, that with this scheme you have to manually free the memory containing the string when you are done with the structure.

Answer 2

Just add a member function that takes in the character buffer(function input parameter char * ) and populates the test structure by parsing it.
This makes it more clear and readable as well.

If you provide a implicit conversion constructor then you create a menace which will do the conversion when you least expect it.

Answer 3

When reading variable length data from a sequence of bytes, you shouldn't fit everything into a single structure or variable. Pointers are also used to store this variable length.

The following suggestion, is not tested:

// data is stored in memory,
// in a different way,
// NOT as sequence of bytes,
// as provided
struct data {
    short sh1;
    short sh2;
    int abclength;
    // a pointer, maybe variable in memory !!!
    char* abc;
    bool bool1;
    bool bool2;
};

// reads a single byte
bool readByte(byte* MyByteBuffer)
{
    // your reading code goes here,
    // character by character, from stream,
    // file, pipe, whatever.
    // The result should be true if not error,
    // false if cannot rea anymore
}

// used for reading several variables,
// with different sizes in bytes
int readBuffer(byte* Buffer, int BufferSize)
{
    int RealCount = 0;

    byte* p = Buffer;

    while (readByte(p) && RealCount <= BufferSize)
    {
        RealCount++
        p++;
    }

    return RealCount;
}

void read()
{
    // real data here:
    data Mydata;

    byte MyByte = 0;

    // long enough, used to read temporally, the variable string
    char temp[64000];
    // fill buffer for string with null values
    memset(temp, '\0', 64000);

    int RealCount = 0;

    // try read "sh1" field
    RealCount = (readBuffer(&(MyData.sh1), sizeof(short)));
    if (RealCount == sizeof(short))
    {
        // try read "sh2" field
        RealCount = readBuffer(&(MyData.sh2), sizeof(short));       
        if (RealCount == sizeof(short))
        {
            RealCount = readBuffer(temp, 64000);
            if (RealCount > 0)
            {
                // store real bytes count
                MyData.abclength = RealCount;
                // allocate dynamic memory block for variable length data
                MyData.abc = malloc(RealCount);
                // copy data from temporal buffer into data structure plus pointer
                // arrays in "plain c" or "c++" doesn't require the "&" operator for address:
                memcpy(MyData.abc, temp, RealCount);

                // comented should be read as:
                //memcpy(&MyData.abc, &temp, RealCount);

                // continue with rest of data
                RealCount = readBuffer(&(MyData.bool1), sizeof(bool));
                if (RealCount > 0)
                {
                    // continue with rest of data
                    RealCount = readBuffer(&(MyData.bool2), sizeof(bool));
                }
            }
        }
    }
} // void read()

Cheers.