c++ using memcpy to copy inside the array

Question

If I have an array of elements, is it okay to use memcpy to copy some data from the back of an array to the front part

Presented in code:

int a[5] = {1,2,3,4,5};

memcpy(a, a + 2, 3 * sizeof(int));

As you can see some parts are copied to the other place in the front and then overwritten later (number 3).

Array after memcpy is {3, 4, 5, 4, 5}, but does this invoke undefined behaviour or is it completely valid to do such thing? I'm using c++ and VS2017 compiler.

»If the objects overlap the behavior is undefined.« en.cppreference.com/w/c/string/byte/memcpy — Henri Menke
– Henri Menke, Commented Jun 12, 2017 at 1:13
thank you henri menke, u can post it as an answer so i can accept it — Sly_TheKing
– Sly_TheKing, Commented Jun 12, 2017 at 1:21

Mark B · Accepted Answer · 2017-06-12 02:42:02Z

1

This is not ok: memcpy requires that the source and destination regions not overlap. C specifically provides the function memmove for cases where the regions do overlap.

Since you're tagged C++ here, you could also consider using std::copy as its constraint is weaker than memcpy: Just the the destination iterator must be outside the range to be copied, which is the case in your scenario.

answered Jun 12, 2017 at 2:42

Mark B

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