Is std::vector so much slower than plain arrays?

Question

I\'ve always thought it\'s the general wisdom that std::vector is \"implemented as an array,\" blah blah blah. Today I went down and tested it, and it seems to

Answer 1

Try with this:

void UseVectorCtor()
{
    TestTimer t("UseConstructor");

    for(int i = 0; i < 1000; ++i)
    {
        int dimension = 999;

        std::vector pixels(dimension * dimension, Pixel(255, 0, 0));
    }
}

I get almost exactly the same performance as with array.

The thing about vector is that it's a much more general tool than an array. And that means you have to consider how you use it. It can be used in a lot of different ways, providing functionality that an array doesn't even have. And if you use it "wrong" for your purpose, you incur a lot of overhead, but if you use it correctly, it is usually basically a zero-overhead data structure. In this case, the problem is that you separately initialized the vector (causing all elements to have their default ctor called), and then overwriting each element individually with the correct value. That is much harder for the compiler to optimize away than when you do the same thing with an array. Which is why the vector provides a constructor which lets you do exactly that: initialize N elements with value X.

And when you use that, the vector is just as fast as an array.

So no, you haven't busted the performance myth. But you have shown that it's only true if you use the vector optimally, which is a pretty good point too. :)

On the bright side, it's really the simplest usage that turns out to be fastest. If you contrast my code snippet (a single line) with John Kugelman's answer, containing heaps and heaps of tweaks and optimizations, which still don't quite eliminate the performance difference, it's pretty clear that vector is pretty cleverly designed after all. You don't have to jump through hoops to get speed equal to an array. On the contrary, you have to use the simplest possible solution.