C++指针——奇怪的循环行为

在main中，我试图调用mymath.cpp中的一个素数函数——它有一些非常奇怪的行为，我不理解。（注意，算法不起作用然而——但这对我来说并不奇怪。）

奇怪的是，如果我评论这句话：

cout << "n:" << lastPrime->pnum <<"n";

在mymath.cpp中，我在main中的循环只运行两次。如果我把它留在里面，我的主循环一直运行到I=50；

MAIN.CPP

#include <iostream>
#include <stdlib.h>
#include <time.h>
#include "stat.h"
#include "mymath.h";
using namespace std;
int main()
{
    for (int i = 3; i<= 50; i++)
    {
        if (isPrime(i))
        {
            cout << i << " is prime!n";
        }
        else
        {
            cout << i << " is NOT primen";
        }
    }
    return 0;
}

MYMATH.CPP

#include "mymath.h"
#include <math.h>
#include <iostream>
using namespace std;
prime two;
prime * lastPrime = &two;
prime * firstPrime = &two;
bool isPrime(long long n)
{
  two.pnum=2;
  prime * currentPrime = &two;
  if ( n < 2)
      return false;
  long long squareRoot = sqrt(n);
  while(true)
  {
      if (n % currentPrime->pnum==0)
      {
          //n is divisible by a prime number, nothing left to do.
          return false;
      }
      else
      {
          //n is not divisible by a prime... check next one
          {
          if (currentPrime->pprime == 0 || currentPrime->pnum > squareRoot)
              {
                  //this is prime
                  prime addPrime;
                  addPrime.pnum=n;
                  addPrime.pprime=0;
                  lastPrime->pprime=&addPrime;
                  lastPrime=&addPrime;
                  cout << "n:" << lastPrime->pnum <<"n";
                  return true;
              }
              else
              {
                  //may not be prime, check next
                  currentPrime = currentPrime->pprime;
              }
          }
      }
  }
  return true;
}