C ++计算大小

Question

Consider a packet which consists of header followed by multiple values. If hdr points to the packet header and lastValue points to last value in the packet. can you please explain me the following code:

size_t  calcSize   = (lastValue - (char *)hdr); 

What size does calcSize denote of the packet. In other words what are the start and end locations that is being considered for this size. Thanks,

Answer 1

calcSize将是数据包中的字节数。

Answer 2

In this case this kind of arithmetic makes not much sense. This is because your last entity/value in a packet may be for example 32bit(4byte) and substracting pointer to a header from it makes no sense - better is to use pointer to first byte after the packet in your substraction

int packetLen = (char*)firstByteAfterPacketPtr - (char*)headerPtr;  

      7   8   9  10  11  12  13  14  15  16   17  <-- example byte index
    +---+---+---+---+---+---+---+---+---+---+
... |   header  |   v1  |v2 |    value3     |......
    +---+---+---+---+---+---+---+---+---+---+
      ^                       ^                ^
      headerPtr               lastValPtr       firstByteAfterPacketPtr

Your arithmetic would give:

 int packetLen = (char*)lastVarPtr - (char*)headerPtr; // 13-7=6 wrong

Correct is:

 int packetLen = (char*)firstByteAfterPacketPtr - (char*)headerPtr; // 17-7=10 ok

Be careful about pointer arithmetic - in case of (char*) you get byte difference, in case of pointers to 16bit eitities you get diff expressed in number of 16bit entities etc..

Answer 3

这是指针算法： (lastValue - hdr)表示两个char指针之间的差，这意味着它们之间的char大小。