C++ string库 resize坑

#include <sys/time.h>

int main()
{
  string str1;
  string str2;
  str1.resize(100000000);
  unsigned long realtime;
  struct timeval ts;
  unsigned long timecount;
  gettimeofday(&ts, NULL);

  timecount = ts.tv_sec * 1000 + ts.tv_usec / 1000;

  cout << "init time:" << timecount << endl;
  for (size_t i = 0; i < 100000000; i++)
  {
    str1.append("1");
  }
  gettimeofday(&ts, NULL);
  realtime = ts.tv_sec * 1000 + ts.tv_usec / 1000;

  cout << "time2:" << realtime - timecount << endl;
  timecount = realtime;

  for (size_t i = 0; i < 100000000; i++)
  {
    str2.append("1");
  }
  gettimeofday(&ts, NULL);
  realtime = ts.tv_sec * 1000 + ts.tv_usec / 1000;
  cout << "time3:" << realtime - timecount << endl;

  // resize() 垃圾方法  没啥用
}

 Resizes the %string to the specified number of characters.
       *  @param  __n  Number of characters the %string should contain.
       *
       *  This function will resize the %string to the specified length.  If
       *  the new size is smaller than the %string's current size the %string
       *  is truncated, otherwise the %string is extended and new characters
       *  are default-constructed.  For basic types such as char, this means
       *  setting them to 0.