Copying std::vector: prefer assignment or std::copy?

Question

I have two vectors:

std::vector<int> v1, v2;

// Filling v1
...

And now I need to copy v1 to v2 . Is there any reason to prefer

v2 = v1;

to

std::copy (v1.begin(), v1.end(), v2.begin());

(or vice versa)?

Answer 1

Generally I would strongly prefer v2 = v1 :

1. It is shorter and makes the intend more clear
2. std::copy won't work if v2 doesn't have the same length as v1 (it won't resize it, so it will retain some of the old elements best case ( v2.size() > v1.size() and overwrite some random data used elsewhere in the program worst case
3. If v1 is about to expire (and you use C++11) you can easily modify it to move the contents
4. Performancewise assignment is unlikely to be slower then std::copy , since the implementers would probably use std::copy internally, if it gave a performance benefit.

In conclusion, std::copy is less expressive, might do the wrong thing and isn't even faster. So there isn't really any reason to use it here.

Answer 2

The invocation of std::copy may try to access items beyond the end of the destination vector.

Use assignment.

It's not your job to micro-optimize: that's the library writer's responsibility, and ultimately the compiler's responsibility.

You can make your code arbitrarily fast if it doesn't have to be correct.

In the case of the copy , however, it's rather doubtful whether it even is faster, and it's certainly not correct for the general case.

Answer 3

If v2 isn't big enough you'll get a buffer overrun if you use copy as you have.

You can use a back insert iterator which will call push_back on v2 . However this could lead to multiple reallocations depending upon how big v1 is.

copy(v1.begin(), v1.end(), back_inserter(v2));

You're better off letting vector manage things correctly. The assignment operator does this, as does vector::assign :

v2.assign(v1.begin(), v1.end());

I have an inkling that the assignment operator is implemented in terms of vector::assign .

Answer 4

It's shorter.

std::copy is mainly meant for copying sections of containers. If you need to copy an entire container, you might as well use the copy constructor.

Answer 5

Assignment, by far. More generally, any time the size of the vector might change, or change the entire contents of the vector, you should prefer member functions. The only time std::copy would be appropriate is if you are only replacing a small range totally within the vector.

Answer 6

Assignement更清晰，内部使用std::copy （或unitizalized_copy _M_allocate_and_copy具体取决于大小和容量），因此性能相同。