C++ Std queue and vector performance

Question

I've been working with graphs lately, and I am looking into returning a path from a graph. The path needs to be returned as a std vector containing all of the nodes with the starting node first.

I've been looking at two options: - use the slow vector insert method to add the nodes at the front of the vector - use a deque to add the nodes to the front (push_front), which is much faster. Then copying the deque to the vector using std::copy

Is there a significant performance boost using one method over the other?

Answer 1

Since you're returning a path, you presumably have an upper bound on its length. Therefore, you can call create a vector , call reserve and later (as @user2079303 writes) call push_back to add vertices to the path.

const auto n = <graph_size>
std::vector<size_t> path;
path.reserve(n)
...
v.push_back(i); // Push whatever you want.

Now the problem is that, at least from the question, it seems like v is in the reversed order. However, you can simply call std::reverse :

std::reverse(std::begin(v), std::end(v));

So, using only a vector :

• You're allocating a single data structure instead of two; moreover, using reserve there will be a single memory allocation.

• The use of reverse at the end simply replaces the use of copy you would have to do from the deque to the vector.

Answer 2

If you are looking at wrapping a std::vector in a std::queue then the std::queue will push elements to the back of the vector (the fast way).

Even if not however, because a std::vector is contiguous storage, it is possible it will out-perform a std::deque even if you push_font() because it plays well with the CPU cache where shuffling data is fast.

But why not try both and profile the code to see which one performs better?