sizeof(struct) returns a SMALLER than expected value?

debugcn Published at Dev

nass

I have this c++ structure:

struct Packet
{
    uint32 MessageCount; 
    uint32 Length; 
    uint32 FieldValue;

    union PacketHeader
    {
        uint32 typeInfo; 
        struct MagicVersion
        {
            uint8 MagicNumber[3];
            uint8 Version; 
        };
     };

    Data * Payload(void) { return reinterpret_cast< Data * >(this + 1U); }
    Data const * Payload(void) const { return reinterpret_cast< Data const * >(this + 1U); }

    Packet * nextPacket(void) { return reinterpret_cast< Packet * >(this + 1U) + Length; }
    Packet const * nextPacket(void) const { return reinterpret_cast< Packet const * >(this + 1U) + Length; }
};

Then sizeof(Packet) in MSVC++ returns 12 instead of 16 which is what I expect.

The weird thing is of course that this is smaller that the expected value. Had it been bigger it could be because of alignment issues.

What am I missing?

TIA

Dutow

Why do you think it should be bigger?

It contains 3 uint32 with no virtual methods, that's exactly 12 bytes.

The union doesn't count, since it's a type, it contains no member of that type.

If you want your class to contain a single instance of the union, which contains a single instance of the struct, you should write:

struct Packet
{
    uint32 MessageCount; 
    uint32 Length; 
    uint32 FieldValue;

    union
    {
        uint32 typeInfo; 
        struct
        {
            uint8 MagicNumber[3];
            uint8 Version; 
        } /* MagicVersion */;
     };
} /* PacketHeader */;

The names within /**/ are optional, you can either specify them or not. If you do, you have to access their members using the name of the union/struct, otherwise you'll have a flat Packet struct.

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