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?

`memmove` is for moving *bytes*, not objects. You are moving 2 bytes. An `int` is (usually) 4 bytes. — BoBTFish, Jan 02 '17 at 11:22
@BoBTFish So I should use 2 * sizeof(int) instead of 2 as the third parameter in memove, am I right? — vinllen, Jan 02 '17 at 11:26
No, you should not use `memmove` to move objects. Consider `std::copy`, but look at the other standard algorithms too. — BoBTFish, Jan 02 '17 at 11:28
By the way, I think the correct term for what you want to achieve is "to copy". — HolyBlackCat, Jan 02 '17 at 11:35
@vinllen "It works" and "it is a good idea" are 2 very different things. — BoBTFish, Jan 02 '17 at 11:53

Emil Laine · Accepted Answer · 2017-01-02T11:31:21.497

4

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);

edited Jan 02 '17 at 11:31

answered Jan 02 '17 at 11:25

Emil Laine

Yes, it's right. It looks like `memmove` using another temporary memory to store the source data which is inefficient – vinllen Jan 02 '17 at 11:33
2

@vinllen In practice, no. Check out the "notes" section here: http://en.cppreference.com/w/cpp/string/byte/memmove. – Emil Laine Jan 02 '17 at 11:37
I still have a question, this post(http://www.cplusplus.com/reference/algorithm/copy_n/) says "If the ranges overlap, some of the elements in the range pointed by result may have undefined but valid values." What's the meaning of "undefined but valid" ? The function template seems it not supports overlapped ranges – vinllen Jan 02 '17 at 11:58
@vinllen The standard doesn't specify the order in which elements are copied. Thus, the result of the operation is implementation-dependent for overlapping ranges. – Joseph Thomson Jan 02 '17 at 13:39

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