Custom copying dynamic array

Question

How can I copy dynamic array like that: Copy all elements of array, but start in second array not from 0 index, but from first.

//We have array x, which have 5 elements, and array y, which have 6 elements;
y[1]=x[0];
y[2]=x[1];
//etc...

P.S. Don't want copy element by element, can I use in that way std::copy or memcpy ??

EDIT: Is it more efficient way to reallocate an array than this?

int* arr = new int[20];
...
//wanna resize to 25?
int* temp = new int[25];
std::copy(arr, arr+20, temp);
delete [] arr;
arr = temp;
... //now arr has 25 elements

But without using vector or other stl objects.

Since we don't know what is *in* your arrays, its hard to say whether `std::copy`, `memcpy`, both, or *neither* is appropriate for your task. `std::copy` would *likely* work, but that assumes whatever type these are is at-least assignable. — WhozCraig, Apr 27 '15 at 06:51

meddle0106 · Accepted Answer · 2015-04-27T07:05:31.420

4

std::copy uses iterators, so you can increment the destination iterator by one:

for fixed size arrays and c++11, this should work:

std::copy( std::begin(x), std::end(x), std::begin(y)+1 )

for dynamic arrays, std::end does not work, but pointer arithmetics can be used:

std::copy( x, x+5, y+1 )

edited Apr 27 '15 at 07:05

answered Apr 27 '15 at 06:53

meddle0106

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`std::begin` is from which library? – bingo157 Apr 27 '15 at 06:58
This is c++11. But you're right, this is not working for dynamic arrays. But you can use raw pointers instead. Someone like: std::copy( x, x+, y+1) should work – meddle0106 Apr 27 '15 at 07:00

WhozCraig · Answer 2 · 2015-04-27T07:18:49.880

You can use either std::copy or any of the appropriate memory copy/move functions (memcpy, memmove if regions overlap) for your situation, the latter because your object type (uint8_t) is trivially-copy-assignable.

You should also know std::copy will likely do the latter for you if your type is trivially copy assignable (and uint8_t is so). This is not a requirement (at least none that I'm aware of, I'm sure I'll hear about it if it is) but each vendor I've seen provides a SFINAE solution for trivially-copy-assignable objects to simply perform a memmove in the interest of efficiency.

In short, you will likely get what you want with std::copy

std::copy(x, x+x_len, y+1);

and make sure y has at least x_len+1 slots available.

Custom copying dynamic array

2 Answers2