C++ 从 int 转换为字节数组，然后将该字节数组转换回 int，导致溢出

Question

I'm making a multiplayer game where I use CSocket to send data between server and clients, and I need to transfer raw bytes. So I tested out how to convert from integer to byte array and reverse, this is what I tested:

int test1 = 257;
byte bytes[4];
copy(&test1, &test1 + 3, bytes);

int test2;
copy(bytes, bytes + 3, &test2);
cout << "Test " << test2 << endl;

The test2 variable when printed out, the value isn't 257 but 1 instead, I think it's because type byte (unsigned char) has size of 256 and it is experiencing an overflow. Is there anyway I can convert properly?

Answer 1

All pointer arithmetic will be done in units of the base type.

For a pointer to int ( int* ) then adding 3 will add a byte-offset of 3 * sizeof(int) .

So the call copy(&test1, &test1 + 3, bytes) will copy 3 int values into the four-byte array.

To copy only one int then add 1 instead:

copy(&test1, &test1 + 1, bytes);

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It might help to see any pointer as a pointer to the first element of an array.

For &test1 it's could then be seen as a pointer to the first element of an array of a single int element.

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And as mentioned in a comment (thanks for pointing it out john) you only copy three bytes from the bytes array into test2 .

The end "iterator" should be one element beyond the end, which is a pointer to bytes[4] . That is you need

copy(bytes, bytes + 4, &test2);

Answer 2

Here's your code fixed

int test1 = 257;
byte bytes[4];
copy((char*)&test1, (char*)&test1 + 4, bytes);

int test2;
copy(bytes, bytes + 4, (char*)&test2);
cout << "Test " << test2 << endl;

I convert all the pointers to char pointers, and I use +4 not +3. +3 only copies three bytes so it fails to copy the last byte.