Efficiently populate vector of known size

Question

I want to create a std::vector<int> of a known size and immediately fill it with some generated values. Is there a way to do this without either (1) first zero-filling the vector to the desired size or (2) using reserve then push_back on every element?

For example, to fill the vector with values 0 though size-1:

// method 1
vector<int> fill_resize(int s) {
  vector<int> v(s);
  for (int i = 0; i < s; i++) {
    v[i] = i;
  }
  return v;
}

// method 2
vector<int> fill_push_back(int s) {
  vector<int> v;
  v.reserve(s);
  for (int i = 0; i < s; i++) {
    v.push_back(i);
  }
  return v;
}

Method (1) wastes time by redundantly zero filling, and method (2) needs the machinery of push_back on every insert, which in practice compiles poorly.

Before someone jumps in and says "compilers are smart, they'll optimize this for you!" - check out the generated assembly on cutting edge compilers at -O2. It is pretty awful, with the push_back variant taking the cake in awfulness¹.

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¹Interestingly enough, the awful push_back version still wins for very large vectors, because there you are limited mostly by memory bandwidth, and the zero-then-fill approach of method 1 takes 2x the bandwidth. The push_back approach at least only loops over the values once, but using a slow loop.

Answer 1

You could:

• Create your own vector type.
• Use unique_ptr as described here (assuming you don't need to resize your array): https://stackoverflow.com/a/24852984/673679
• Fill your vector with a proxy integer type, with an empty constructor, similar to what's described here: https://stackoverflow.com/a/15220853/673679