[c++] Using arrays or std::vectors in C++, what's the performance gap?

Vectors are arrays under the hood. The performance is the same.

One place where you can run into a performance issue, is not sizing the vector correctly to begin with.

As a vector fills, it will resize itself, and that can imply, a new array allocation, followed by n copy constructors, followed by about n destructor calls, followed by an array delete.

If your construct/destruct is expensive, you are much better off making the vector the correct size to begin with.

There is a simple way to demonstrate this. Create a simple class that shows when it is constructed/destroyed/copied/assigned. Create a vector of these things, and start pushing them on the back end of the vector. When the vector fills, there will be a cascade of activity as the vector resizes. Then try it again with the vector sized to the expected number of elements. You will see the difference.

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