Order of execution for `::memcpy` [duplicate]

Question

This question already has answers here:

memcpy() vs memmove() (11 answers)

Closed 9 years ago.

I'm writing a class member function to remove a row from a (m x n) matrix. The memory model for the matrix (from a standard linear algebra library) is guaranteed to be a contiguous block such that the (i, j)th element maps to the (i * n + j)th element of that memory block.

I'd like to use ::memcpy for clarity and performance reasons. (The alternative ways using for loops are cumbersome.) But I need some reassurance.

In general, the destination location overlaps the source location since, if I'm deleting row r, I'd use (dropping any casts for clarity),

::memcpy(r * n, (r + 1) * n, bigger than n in general)

If ::memcpy is guaranteed to copy from the start of the block, then this will be fine. But, does the standard guarantee this? I can't see why it would and suspect the behaviour of such code is undefined.

Answer 1

You should use memmove instead of memcpy. The overlapping behaviour for memmove is defined.

User thang pointed out in a comment, there is a related article which dicusses the difference between memcpy and memmove.

Answer 2

Have you considered using std::copy or std::copy_backwards?