Is there any way to control the padding between struct members (incl. bit field) in C++?

Question

I'm working on parsing network data stream, I'm wondering if there is any way I can map the data stream directly to the data structure.

For example, I want to define the data structure for an RTP protocol as follows.

class RTPHeader
{
   int version:2; // The first two bits is version.
   int P:1;  // The next bits is an field P.
   int X:1;
   int CC:4;
   int M:1;
   int PT:7;
   int sequenceNumber;
   int64 timestamp;
   .....
};

And using it this way.

RTPHeader header;
memcpy(&header, steamData, sizeof(header));

But as the C++ compiler will insert padding between the members, is there any way to control that so that no padding are added between members (including the bit field members)?

This question is NOT a duplicate of How to get rid of padding bytes between data members of a struct because there can be bit fields in my example.

Answer 1

If you're able to use C++11, you can take advantage of the align control implemented with the alignof operator.

If you cannot use a C++11 compiler, there's non-standard alternatives that can help you; in GCC the __attribute__(packed), and with MSVC the #pragma pack.

If your choice is the GCC variant, the attribute must be placed at the end of the struct:

class RTPHeader
{
    int version:2; // The first two bits is version.
    int P:1;  // The next bits is an field P.
    int X:1;
    int CC:4;
    int M:1;
    int PT:7;
    int sequenceNumber;
    int64 timestamp;
    .....
} __attribute__((packed)) ; // attribute here!

If your choice is the MSVC one, the pragma must be placed before the struct:

#pragma pack(1) // pragma here!
class RTPHeader
{
    int version:2; // The first two bits is version.
    int P:1;  // The next bits is an field P.
    int X:1;
    int CC:4;
    int M:1;
    int PT:7;
    int sequenceNumber;
    int64 timestamp;
    .....
};

If your code must be compiled in both, the only way (without C++11 alignof operator) is the conditional compilation:

#ifdef MSVC
#pragma pack(1)
#endif
class RTPHeader
{
    int version:2; // The first two bits is version.
    int P:1;  // The next bits is an field P.
    int X:1;
    int CC:4;
    int M:1;
    int PT:7;
    int sequenceNumber;
    int64 timestamp;
    .....
#ifdef GCC
}__attribute__((packed));
#else
};
#endif