Which stl container should I use when doing few inserts?

Question

I don't know my exact numbers but i'll try my best. I have a 10000 element deque thats populated right at the start. Than i scan through each element and lets every 20 elements i'll need to insert an new element. The insert would happen at the current position and maybe one element back.

I don't exactly need to remember the position but i also don't exactly need random access either. I'd like fast inserts. Does deque and vector have a heavy price to pay on insert? Should i use list?

My other option is to have a 2nd deque list and as i go through each element insert it to the other deque list unless i need to do the insert i am talking about. This does need to be fast as its a performance intensive app. But I am using a lot of pointers (each element is a pointer) which is upsetting me but there isn't a way around that so i should assume L1 cache will always miss?

Answer 1

I'd start with std::vector in this case, but use a second std::vector for your mass mutations, reserve() appropriately, then swap() the vectors.

Update

It would take this general form:

std:vector<t_object*> source; // << source already holds 10000 elements

std:vector<t_object*> tmp;

// to minimize reallocations and frees to 1 and 1, if possible.
// if you do not swap or have to grow more, reserving can really work against you.
tmp.reserve(aMeaningfulReserveValue);

while (performingMassMutation) {
  // "i scan through each element and lets every 20 elements"
  for (twentyElements)
    tmp.push_back(source[readPos++]);

  // "every 20 elements i'll need to insert an new element"
  tmp.push_back(newElement);
}

// approximately 500 iterations later…

source.swap(tmp);

Borealid brought up a good point, which is measure -- execution varies dramatically depending on your std library implementations, data sizes, complexity to copy, and so on.

For raw pointers of a collection this size with my configuration, the vector mass mutation and push_back above was 7 times faster than std::list insertion. push_back was faster than vector's range insertion.

As Emile points out below, std::vector::swap() does not need to move or reallocate elements -- it can just swap out internals (provided the allocators are the same type).

Answer 2

First off, the answer to all performance questions is "benchmark it". Always. Now...

If you don't care about the memory overhead, and you don't need random access, but you do care about having constant-time insertions, list is probably right for you.

std::vector will have constant-time insertions at the end when it has sufficient capacity. When the capacity is exceeded, it needs a linear-time copy. deque is better because it links discrete allocations, avoiding a complete copy and letting you do constant-time insertions at the front as well. Random insertions (every 20 elements) will always be linear time.

As for cache locality, a vector is as good as you can get (contiguous memory), but you said you cared about insertions rather than lookups; in my experience, when that's the case you don't care about how hot the cache gets as you scan through to dump, so list's poor behavior doesn't much matter.

Answer 3

The number of copies done for std::vector/deque ::insert etc is proportional to the number of elements between the insert position and the end of container (the number of elements that need to be shifted to make room). The worst-case for a std::vector is O(N) - when you insert at the front of the container. If you're inserting M elements the worst -case is therefore O(M*N) which isn't great.

There could also be a reallocation involved if the containers capacity is exceeded. You could prevent reallocation by ensuring that sufficient space was ::reserve'd up front.

You're other suggestion - copying to a second std::vector/deque container could be better in that it could always be organised to achieve O(N) complexity, but at the cost of temporarily storing two containers.

Using a std::list would allow you to achieve in-place O(1) inserts, but at the cost of additional memory overhead (storing the list pointers etc) and reduced memory locality (list nodes are not allocated contiguously). You could improve the memory locality by using a pool'd memory allocator (Boost pools maybe?).

Overall you'd have to benchmark to really sort out which is "the fastest" approach.

Hope this helps.

Answer 4

If you need fast inserts in the middle, but don't care about random access, vector and deque are definitely not for you: For those, every time you insert something, all elements between that one and the end have to be moved. Of the built-in containers, list is almost certainly your best bet. However a better data structure for your scenario would probably be a VList because it provides better cache locality, however that's not provided by the C++ standard library. The Wikipedia page links to a C++ implementation, however from a quick view on the interface it doesn't seem to completely STL compatible; I don't know if this is an issue for you.

Of course, in the end the only way to be sure which is the optimal solution is to measure the performance.

Answer 5

You can go two ways: list is always an option for random place insertions, however as you allocate every element separately this will cause some performance implications too. The other option of inserting in-place in the deque is not good as well - because you will pay linear time for every insertion. Maybe your idea of inserting in new deque is the best here - you pay twice as much memory, but on the other hand you always do insertion either at the end of the second deque, or one element before that - this all gives constant amortized time, and still you have good caching of the container.

Answer 6

Lists are useful when either you frequently want to insert elements in the middle of the collection, or frequently remove them. Lists are, however, slow to read.

Vectors are very fast to read and very fast when you only want to add or remove elements at the end of the collection, but they are very slow when you insert elements in the middle. This is because it has to move all elements after the desired position by one place, to make room for the new element.

Deques are basically doubly linked lists that can be used as vectors.

If you don't need to insert elements in the middle of the collection (you don't care about the order), I suggest you use vector. If you can approximate the number of elements that will be introduced in the vector from the beginning, you should also use std::vector::reserve to allocate memory necessary from the beginning. The value you pass to reserve doesn't need to be exact, just approximate; if it's smaller than needed, the vector will resize automatically, when necessary.