Fast vector generation (>= C++11)

fast_vec_gen.cpp

#include <string>
#include <vector>

void main() {
  std::string foo = "baz";
  std::vector<char> bar;
  printf("%d\n", bar.capacity());
  bar.resize(foo.size());
  // CAUTION: vector::reserve affects only "capacity", so does not change its "size",
  // will causes empty-like behavior if using for(:) loop, vector::resize is adequate.
  // bar.reserve(foo.size());
  printf("%d\n", bar.capacity());
  // vector::reserve() and memcopy(vector::data(), ...) will be faster
  // than push_back for-loop, if not data conversion is not required.
  memcpy(bar.data(), foo.data(), foo.size());
  printf("bar: %x %x %x \n", bar[0], bar[1], bar[2]);
  printf("foo: %x %x %x \n", foo[0], foo[1], foo[2]);
}

// RESULT:
// 0
// 3
// bar: 62 61 7a
// foo: 62 61 7a

fast_vec_gen_comp.cpp

#include <string>
#include <vector>
#include <chrono>

std::vector<char> fast(std::string data);
std::vector<char> slow(std::string data);

void main() {
  std::string data = "The quick brown fox jumps over the lazy dog.";
  const char* unit = "microseconds";
  const int e6 = 1000000;
  std::chrono::high_resolution_clock::time_point t1 = std::chrono::high_resolution_clock::now();
  for (int c = 0; c < e6; c++)
    slow(data);
  std::chrono::duration<double> d1 = std::chrono::high_resolution_clock::now() - t1;
  std::chrono::high_resolution_clock::time_point t2 = std::chrono::high_resolution_clock::now();
  for (int c = 0; c < e6; c++)
    fast(data);
  std::chrono::duration<double> d2 = std::chrono::high_resolution_clock::now() - t2;
  printf("SLOW: %f %s\nFAST: %f %s\n", d1.count(), unit, d2.count(), unit);
}
// ** FAST METHOD IS 10-TIMES FASTER **
// SLOW: 5.445815 microseconds
// FAST: 0.549105 microseconds
std::vector<char> fast(std::string data) {
  std::vector<char> v;
  v.resize(data.length());
  memcpy(v.data(), data.data(), data.size());
  return v;
}
std::vector<char> slow(std::string data) {
  std::vector<char> v;
  for (int c = 0, l = data.length(); c < l; c++)
    v.push_back(data[c]);
  return v;
}