Is it more efficient to preallocate a vector?

Question

In C++ Primer fourth edition, by Stanley B.Lippman, Josee Lajoie and Barbara E. Moo it states:

Because vectors grow efficiently, it is usually best to

Answer 1

I timed this simple example:

#include
#include

int main() {

    int limit = 100 * 1000 * 1000;
    std::vector my_vec;
    my_vec.reserve(limit); // comment out this line to not preallocate

    for (int i=0; i < limit; i++) {
        my_vec.push_back(i);
    }

    long my_sum = 0;
    for (int i=0; i < limit; i++) {
        my_sum += my_vec[i];
    }

    std::cout << my_sum << std::endl;
    return 0;
}

Complied with:

g++ -std=c++11 -O2 my_file.cpp -o my_exec

And found the difference to be substantial:

Without preallocation:

real    0m3.366s
user    0m1.656s
sys     0m1.660s

With preallocation:

real    0m1.688s
user    0m0.732s
sys     0m0.936s

My conclusion here is: If building a vector is a big part of the program, then preallocating for efficiency makes sense. However, building a larger vector over and over is unlikely, and thus it is rarely a bottle neck. However, using reserve() has other advantages besides preallocating.

Bjarne Stroustrup in The C++ programming language (4th addition) has this to say:

I used to be careful about using reserve() when I was reading into a vector. I was surprised to find that for essentially all my uses, calling reserve() did not measurably affect performance. The default growth strategy worked just as well as my estimates, so I stopped trying to improve performance using reserve(). Instead I use it to increase predictability of reallocation delays and to prevent invalidation of pointers and iterators.