c++ std::vector<> vs new[] performance

Question

Is there any big difference in allocation, deallocation and access time between std::vector<> and new[] when both are fixed and same length?

Answer 1

Depends on the types and how you call it. std::vector<int> v(1000000); has to zero a million ints, whereas new int[1000000]; doesn't, so I would expect a difference in speed. This is one place in std::vector where you might pay through the nose for something you don't use, if for some reason you don't care about the initial values of the elements.

If you compare std::vector<int> v(1000000); with new int[1000000](); then I doubt you'll see much difference. The significant question is whether one of them somehow has a more optimized loop setting the zeros, than the other one does. If so, then the implementation of the other one has missed a trick (or more specifically the optimizer has).

Answer 2

new is a bad thing, because it violates the idiom of single responsibility by assuming two respon­sibi­li­ties: Storage allocation and object construction. Complexity is the enemy of sanity, and you fight com­plexity by separating concerns and isolating responsibilities.

The standard library containers allow you to do just that, and only think about objects. Moreover, std::vector addionally allows you to think about storage, but separately, via the reserve/capacity interfaces.

So for the sake of keeping a clear mind about your program logic, you should always prefer a container such as std::vector:

std::vector<Foo> v;

// make some storage available
v.reserve(100);

// work with objects - no allocation is required
v.push_back(x);
v.push_back(f(1, 2));
v.emplace_back(true, 'a', 10);