将向量值复制到 char*

Question

I have three vectors

std::vector<long long unsigned> eightByte_v;
std::vector<unsigned char> oneByteDelta_v;
std::vector<unsigned short> twoByteDelta_v;

and I inserted two values each to vectors

eightByte_v.pushback(0x111111111111);
eightByte_v.pushback(0x2222222222222222);
oneByteDelta_v.pushback(0x33);
oneByteDelta_v.pushback(0x44);
twoByteDelta_v.pushback(0x5555);
twoByteDelta_v.pushback(0x6666);

I would like to merge all the vectors to char* as follow.

    char[0] = 00
    char[1] = 00
    char[2] = 11
    char[3] = 11
    char[4] = 11
    char[5] = 11
    char[6] = 11
    char[7] = 11
    char[8] = 22
    char[9] = 22
    char[10] = 22
    char[11] = 22
    char[12] = 22
    char[13] = 22
    char[14] = 22
    char[15] = 22
    char[16] = 22
    char[17] = 22
    char[18] = 33
    char[19] = 44
    char[20] = 55
    char[21] = 55
    char[22] = 66
    char[23] = 66

from char 0 to 17 is equivalent to eightByte_v
from char 18 to 19 is equivalent to oneByte_v
from char 20 to 23 is equivalent to twoByte_v

I think I can do it with memcpy , but is there any other way to do it?

Thank you in advance!

Answer 1

Alternatives like using a union spring to mind, but it's difficult or impossible to do that in a defined way: you'll invariably encounter structure packing and pointer casting issues.

Given that the C++ standard guarantees data contiguity in a std::vector (and the data buffer is accessible using the data() function), a memcpy is arguably the best approach.

Do bear in mind though that the signed-ness of char is platform dependent. You might want to be more explicit; consider using an unsigned char .