应该还是cache的问题，随便写了一个跑了一下，总时间肯定还是不初始化的最好。

#include <chrono>
#include <functional>
#include <iostream>

namespace {
constexpr int POPULATION = 10000000;

struct R3 {
  R3() = default;
  R3(double x, double y, double z) {
    data[0] = x;
    data[1] = y;
    data[2] = z;
  }
  double data[3];
};

void report(const std::function<void()> &f1, const std::function<void()> &f2, bool run_f2_again = false) {
  auto start = std::chrono::high_resolution_clock::now();
  f1();
  auto end = std::chrono::high_resolution_clock::now();
  std::cout << "Allocation done in " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start) << std::endl;

  start = std::chrono::high_resolution_clock::now();
  f2();
  end = std::chrono::high_resolution_clock::now();
  std::cout << "Assignment done in " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start) << std::endl;

  if (run_f2_again) {
    start = std::chrono::high_resolution_clock::now();
    f2();
    end = std::chrono::high_resolution_clock::now();
    std::cout << "Assignment again in " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start) << std::endl;
  }

  std::cout << std::endl;
}

void func1() {
  std::vector<R3> rr;
  report([&rr]() { rr.reserve(POPULATION); },
         [&rr]() {
           for (int i = 0; i < POPULATION; ++i) {
             rr[i] = {(double) i * 2, (double) i + 4, (double) i + 6};
           } });
}

void func2() {
  std::vector<R3> rr;
  report([&rr]() { rr.assign(POPULATION, R3{0, 0, 0}); },
         [&rr]() {
           for (int i = 0; i < POPULATION; ++i) {
             rr[i] = {(double) i * 2, (double) i + 4, (double) i + 6};
           } });
}

void func3() {
  std::vector<R3> rr;
  report([&rr]() { rr.assign(POPULATION, R3()); },
         [&rr]() {
           for (int i = 0; i < POPULATION; ++i) {
             rr[i] = {(double) i * 2, (double) i + 4, (double) i + 6};
           } });
}
}// namespace

int main() {
  func1();
  func2();
  func3();
  return 0;
}


【 在 ziqin 的大作中提到: 】
: 但是他一开始的问题是：
: vector中初始化与否导致性能差异巨大
:
: ...................
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FROM 222.129.53.*