Efficient linked list in C++?

Question

This document says std::list is inefficient:

std::list is an extremely inefficient class that is rarely useful. It performs a heap allocation

Answer 1

Thanks for all the answers. This is a simple - though not rigorous - benchmark.

// list.cc
#include 
using namespace std;

int main() {
    for (size_t k = 0; k < 1e5; k++) {
        list ln;
        for (size_t i = 0; i < 200; i++) {
            ln.insert(ln.begin(), i);
            if (i != 0 && i % 20 == 0) {
                ln.erase(++++++++++ln.begin());
            }
        }
    }
}

and

// vector.cc
#include 
using namespace std;

int main() {
    for (size_t k = 0; k < 1e5; k++) {
        vector vn;
        for (size_t i = 0; i < 200; i++) {
            vn.insert(vn.begin(), i);
            if (i != 0 && i % 20 == 0) {
                vn.erase(++++++++++vn.begin());
            }
        }
    }
}

This test aims to test what std::list claims to excel at - O(1) inserting and erasing. And, because of the positions I ask to insert/delete, this race is heavily skewed against std::vector, because it has to shift all the following elements (hence O(n)), while std::list doesn't need to do that.

Now I compile them.

clang++ list.cc -o list
clang++ vector.cc -o vector

And test the runtime. The result is:

  time ./list
  ./list  4.01s user 0.05s system 91% cpu 4.455 total
  time ./vector
  ./vector  1.93s user 0.04s system 78% cpu 2.506 total

std::vector has won.

Compiled with optimization O3, std::vector still wins.

  time ./list
  ./list  2.36s user 0.01s system 91% cpu 2.598 total
  time ./vector
  ./vector  0.58s user 0.00s system 50% cpu 1.168 total

std::list has to call heap allocation for each element, while std::vector can allocate heap memory in batch (though it might be implementation-dependent), hence std::list's insert/delete has a higher constant factor, though it is O(1).

No wonder this document says

std::vector is well loved and respected.

EDIT: std::deque does even better in some cases, at least for this task.

// deque.cc
#include 
using namespace std;

int main() {
    for (size_t k = 0; k < 1e5; k++) {
        deque dn;
        for (size_t i = 0; i < 200; i++) {
            dn.insert(dn.begin(), i);
            if (i != 0 && i % 20 == 0) {
                dn.erase(++++++++++dn.begin());
            }
        }
    }
}

Without optimization:

./deque  2.13s user 0.01s system 86% cpu 2.470 total

Optimized with O3:

./deque  0.27s user 0.00s system 50% cpu 0.551 total