Thread: Packing buffer to send it (casting)

Suppose that I have a class that has only int, char and so on, I mean, does not has any pointer or class. Could I use the reinterpret class to pack it to send through a network? My problem is that the sizeof operator, used to limit the buffer size, does not alway return the true size of the class. Example:

Code:

class Z{
public:
   //...
private:
   int a;
   char b;
};

int main(){

   char* buffer;
   Z myClass;
   int size = sizeof(Z); //here the size will be 8, not 5

   buffer = new char[size];
   buffer = reinterpret_cast<char *>(&myClass);

   //any send routine
   //send(buffer, size, slave);

   return 0;
}

Thanks any help!

The sizeof operator is working correctly, because a pointer to any type is 32 bits because of the memory address it will store. So you have a 32 bit pointer and a 32 bit integer,which is 64 bits all together. Therefore, 64 / 8 (bits) == 8 bytes

It is returning the true size of the class. They are padding to a 4-byte boundary so your int takes up 4 then your char takes up 1 but is padded to 4. If you're using VC++ you can use pragma to pack it to it's true size on 1 byte boundaries.

#pragma pack(push,1) // 1-byte boundaries

// struct...

#pragma pack(pop) // return to previous packing size

Check your compiler documentation to see if it supports that pragma.

Code:

#include <iostream>

using namespace std;

class Z{
public:
   //...
private:
   int a;
   char b;
};

int main(){

   char* buffer;
   Z myClass;
   int size = sizeof(Z); //here the size will be 8, not 5

   buffer = new char[size];
   buffer = reinterpret_cast<char *>(&myClass);

   cout << "Sending buffer of size " << size << endl;
   
   //any send routine
   //send(buffer, size, slave);

   cin.get();

   return 0;
}

Thank you MrWizard. Unfortunately there aren´t any portable methods to use this idea :-(
But, if the programmer must program the pack function anyway, he could calcute the total size by his own. However this is also not portable :-|

Assuming it's padding at the end, you can probably do this on the receive side:

Z *myClass;
myClass = reinterpret_cast<Z *>(buffer);

Thank you all! Really helped!!!