What's the difference between std::vector and dynamic allocated array?

Question

I have wrote two functions to compare the time cost of std::vector and dynamic allocated array

#include <iostream>
#include <vector>
#include <chrono>

void A() {
  auto t1 = std::chrono::high_resolution_clock::now();
  std::vector<float> data(5000000);
  auto t2 = std::chrono::high_resolution_clock::now();

  float *p = data.data();
  for (int i = 0; i < 5000000; ++i) {
    p[i] = 0.0f;
  }
  auto t3 = std::chrono::high_resolution_clock::now();
  std::cout << std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1).count() << " us\n";
  std::cout << std::chrono::duration_cast<std::chrono::microseconds>(t3 - t2).count() << " us\n";
}

void B() {
  auto t1 = std::chrono::high_resolution_clock::now();
  auto* data = new float [5000000];
  auto t2 = std::chrono::high_resolution_clock::now();
  float *ptr = data;
  for (int i = 0; i < 5000000; ++i) {
    ptr[i] = 0.0f;
  }
  auto t3 = std::chrono::high_resolution_clock::now();
  std::cout << std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1).count() << " us\n";
  std::cout << std::chrono::duration_cast<std::chrono::microseconds>(t3 - t2).count() << " us\n";
}

int main(int argc, char** argv) {
  A();
  B();
  return 0;
}

A() cost about 6000 us to initialize the vector, then 1400 us to fill zeros.

B() cost less than 10 us to allocate memory, then 5800 us to fill zeros.

Why their time costs have such a large difference?

compiler: g++=9.3.0

flags: -O3 -DNDEBUG

Answer 1

First, note that the std::vector<float> constructor already zeros the vector.

There are many plausible system-level explanations for the behavior you observe:

One very plausible is caching: When you allocate the array using new, the memory referenced by the returned pointer is not in the cache. When you create a vector, the constructor will zero the allocated memory area under the hood thereby bringing the memory to the cache. Subsequent zeroing will hit in the cache thus.

Other reasons might include compiler optimizations. A compiler might realize that your zeroing is unneccesary with std::vector. Given the figures you obtained I would discount this here though.