How to manually assign vector's size?

Question

In my application there's a part that requires me to make a copy of a container. At the moment I use std::vector (but might consider using something else). The application is very sensitive with regard to the latency. So, I was looking for a way to make a copy of a vector as fast as possible. I found that memcpy does it better than anything else. However, it does not change the internal size of the vector. I.e. vector.size() would still give me 0.

I know what slippery path I am on. I do not mind throwing away safety checks. I do know how many elements are being copied. I do not want to use vector.resize() to change the size of the destination vector (it is a slow operation).

Question:

std::vector<my_struct> destination_vector;
destination_vector.reserve(container_length);
std::memcpy(destination_vector.data(), original_vector.data(), sizeof(my_struct)*container_length);

After the code above I need to tell my destination_vector what size it is. How do I do this?

Thank you.

Answer 1

1. You must to actually resize() the vector before you copy stuff to it using memcpy():

   destination_vector.resize(container_length);

   But it would be better to avoid the use of memcpy() in the first place and use the mechanisms to copy vector content which is offered by vector, as suggested in the other answers:

   std::vector<my_struct> destination_vector(original_vector);

   or if the destination_vector instance already exists:

   destination_vector.insert(destination_vector.begin(), original_vector.begin(), original_vector.end);

   or, the fastest if you do not need the original content any more:

   destination_vector.swap(original_vector);

   All of these variants will be as fast or even faster than your memcpy()variant. If you experience slowness then see 2.:

2. You probably have a non-trivial default constructor in my_struct. Remove it, or insert a trivial (empty) default constructor to speed things up (to avoid construction of many elements which you never use).

3. If my_structcontains non-POD data members (like std::string) you cannot use memcpy() at all.

(Side note: You rarely want to call reserve(). The vector maintains its own internal storage in such a way that is always allocates more than is actually needed, exponentially, to avoid frequent resizes/copying when frequently appending elements.)

resize() is not a slow operation. It is as fast as any memory allocation.

Does my_struct have a non-trivial default constructor? Remove it and take care of initialization manually. This might be the reason why you say resize() is slow. It will actually construct your objects. But since you can apparently memcpy() your objects you can probably get away with a trivial (empty) default constructor.

Answer 2

Your snippet has undefined behaviour, you can't memcpy into an empty vector, even if you have reserved space. It may also be undefined behaviour to memcpy any my_struct objects, if it isn't a TriviallyCopyable type.

You can construct the vector as a copy of the source directly. Most likely your compiler will emit code identical (or faster) than your original snippet, if my_struct is TriviallyCopyable.

std::vector<my_struct> destination_vector(original_vector.begin(), original_vector.begin() + container_length);

Answer 3

How to manually assign vector's size?

You can't. You can only modify vector's size through the modification functions that add or remove elements such as insert, clear, resize etc.

After the code above I need to tell my destination_vector what size it is. How do I do this?

The mentioned code above has undefined behaviour, so it doesn't matter what you do after it.

A simple and efficient way to copy a vector is:

std::vector<my_struct> destination_vector(original_vector);