使用struct中的位字段并使用C ++从内存中读取实例

Question

unsigned __int16 var16bit= 6545; //00011001 10010001
unsigned __int8 var8bit;
memcpy(&var8bit, &var16bit, sizeof(var8bit));
cout << var8bit; // result is 145 which is 10010001

When working with little endian machine, lsb is written to memory first then offset is added to address if necessary. In this example a 16bit integer is copied to 8bit integer and it losts its msb side because of target space is not enough. Reverse behaviour happens in big endian machines.

There is another example below

here is my structure

struct Account 
{
   unsigned int dollar : 4;
   unsigned int euro : 4;
   unsigned int pound : 4;
   unsigned int ruble : 4;
};

int _tmain(int argc, _TCHAR* argv[])
{
    Account myWealthyAccount = {};
    myWealthyAccount.dollar = 2; // 0010
    myWealthyAccount.euro = 3; // 0011
    myWealthyAccount.pound = 4; // 0100
    myWealthyAccount.ruble = 5; // 0101

    unsigned __int16 sum;
    memcpy(&sum, &myWealthyAccount, sizeof(myWealthyAccount));
    cout << sum;
    // result is 0101010000110010  ruble pound euro dollar
    unsigned __int8 sum8Bit;
    memcpy(&sum8Bit, &myWealthyAccount, sizeof(sum8Bit));
    cout << sum8Bit; // result is 50 which is 0011 0010 euro dollar

    return 0;
}

0101010000110010 ruble pound euro dollar

Why was it ordered as ruble pound euro dollar why not the opposite way?

Why is ruble is beside msb and dollar is beside lsb?

Is this about being little endian or big endian machine? Or Is this about Struct? Is this about compiler?

If I run this example in big endian machine, what result would occur? Why?

I'm open to every advises like articles, videos, websites about the topic.

Thanks

Answer 1

"Why was it ordered as ruble pound euro dollar why not the opposite way?"

I copied your code and got the exact same result. On your first example the 16 bit integer is stored like that 00011001 10010001. So if you take only the lsb you get 10010001. But your Bitfield is stored like 0101 0100 0011 0010.

If you look on http://en.cppreference.com/w/cpp/language/bit_field under notes: "Also, on some platforms, bit fields are packed left-to-right, on others right-to-left" . So it is implementation defined which order you have.