Re: Vectors vs Arrays performance

James Kanze <>
Mon, 4 Jan 2010 14:16:50 -0800 (PST)
On Jan 3, 1:08 pm, "Leigh Johnston" <> wrote:

We can come up with examples and counter-examples all day long.

Exactly. That's what I've been saying from the beginning. You
can't say absolutely that there is an extra access. There might
be. There might not be. It all depends on the actual use, the
compiler and the underlying architecture.

Consider this:

int main()
        std::vector<int> v;
        lib::vecarray<int, 1000> va;
                timer t("vector: ");
                for (int i = 0; i != 1000000; ++i)
                        for (int j = 0; j != 1000; ++j)
                timer t("vecarray: ");
                for (int i = 0; i != 1000000; ++i)
                        for (int j = 0; j != 1000; ++j)

Which outputs (several runs done, VC++ release build, secure
STL disabled):

vector: 2.9741 seconds
vecarray: 2.6782 seconds

Which indicates (for this example) that a stack based container
( is 11% quicker than std::vector.

For this example, with this compiler, on this machine. In
practice, I've had similar results with Sun CC on a Sparc, so
your measurements aren't unique. But in this case, you're
measuring the time necessary to grow the array, which is not the
same thing as the time necessary for an access.

If you'll remember, your original statement was: "Accessing a
vector's contents via operator[] will always be slightly slower
than accessing a local (stack based) array via [] as you have to
dereference a pointer with the vector version." That's the only
statement I'm disagreeing with, and in that statement, what I'm
really disagreeing with is the *always*. Because in most cases,
I think that the "dereference a pointer" simply won't be present
in the individual accesses. Or it will be present with the
"stack based array", because you'll have passed it to a
function. Your statement simply doesn't correspond to the
reality in actual programs.

James Kanze

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