有关打包结构的意外编译警告

Question

当我运行以下代码片段时，收到警告，不知道为什么。 g ++有4.8.4版本。

$ g++ -g  -fPIC -std=c++11 -fpermissive te.cc
te.cc:25:11: warning: ignoring packed attribute because of unpacked non-POD field ‘cMacAddr strArp::src_hw_mac’ [enabled by default]
  cMacAddr src_hw_mac;
           ^
te.cc:27:11: warning: ignoring packed attribute because of unpacked non-POD field ‘cMacAddr strArp::dst_hw_mac’ [enabled by default]
  cMacAddr dst_hw_mac;

代码段“te.cc”

#include <iostream>
#include <utility>
#include <functional>
#include <string>

using namespace std;
typedef unsigned char uchar;
typedef unsigned short uint16;
typedef unsigned int uint;

char _ethHdrPrintBuf[100];

class cMacAddr {
public:
    uchar addr[6];
    cMacAddr() { for (int i=0; i<6; i++) addr[i] = 0;}
};

struct strArp {
    uint16 hw_type;
    uint16 proto_type;
    uchar  hw_size;
    uchar  proto_size;
    uint16 opcode;
    cMacAddr src_hw_mac;
    uint  src_proto_ipv4;
    cMacAddr dst_hw_mac;
    uint  dst_proto_ipv4;
} __attribute__((__packed__ )) ;
int main()
{
    cout << sizeof(strArp) << endl;
    return 0;
}

Answer 1

cMacAddr有一个构造函数，因此它不能成为POD。细节太多here。