How to move 2 elements from head into given position in vector

Question

I want to move the first 2 elements to given position in vector, the result is not right by using memmove in following code:

vector<int> v{1, 2, 3, 4, 0};
memmove(&(v[3]), &(v[0]), 2);

The result by doing so is 1, 2, 3, 1, 0, while the expectation is 1, 2, 3, 1, 2. How can I achieve my job?

Answer 1

memmove copies bytes, not arbitrary objects (like int). So you would need to calculate the correct byte count with 2 * sizeof(int).

But a better way is to use std::copy_n:

std::copy_n(v.begin(), 2, v.begin() + 3);