Floor isn't returning the correct value in C++ [duplicate]

I was attempting a question on SPOJ in which i was required to precompute Cube Root. I approached via binary search method but I am getting wrong answer while checking if given no. is integer or double.

I followed this

if (ceil(num) == floor (num)) //integer

but I am getting wrong answer in floor (num),
for example: 27 (floor = 2, ceil = 3), 125 (floor = 4, ceil = 5)

Here is the code.

#include <iostream>
#include <iomanip>
#include <cmath>
#include <vector>
#define Size 1000001

//vector for storing positions 
std::vector<int> pos;

//function for calculating cube root
double cubeRoot(int n) { 
  double low = 0; 
  double high = (double)n;

  for (int i = 0; i < 50; ++i) { 
    double mid = (low + (high - low) / 2.0);
    if (mid * mid * mid > n) high = mid; 
    else low = mid; 
  }

  //std::cout << " low : " << low << " , high : " << high << '\n';
  return low;
}

int main () { 
  std::ios_base::sync_with_stdio(0);
  std::cout.tie(0);
  std::cin.tie(0);

  //resized according to the question requirement 
  pos.resize(Size, 0);
  pos[1] = 1;

  int currentPosition = 2;  //variable to store the current position. 
  for (int i = 2; i <= Size; i++) { 
    double ans = cubeRoot(i);

    //std::cout << i << ' ' << std::setprecision (10) << ans << ' ' << floor(ans) << ' ' << ceil(ans) << '\n';
    if (ceil (cubeRoot(i)) == floor (cubeRoot(i))) {
      pos[i] = -1; 
      currentPosition --;
    }
    else {
      pos[i] = 1 + currentPosition;
      currentPosition ++;
    }
  }

  int testCase;
  std::cin >> testCase;

  for (int test = 1; test <= testCase; ++test) { 
    int num; 
    std::cin >> num; 

    //Output format
    std::cout << "Case " << test << ": ";
    if (pos[num] == -1) std::cout << "Not Cube Free\n";
    else std::cout << pos[num] << '\n';
  }
}