pow ( abs ( val ), 1.0 / 3.0 )

#include <cstdlib>
#include <cmath>
#include <iostream>
 
int main()
{
  int val;
 
  std::cout<<"Enter a number: ";
 
  if ( std::cin>> val ) {
    std::cout<<"Cube root of "<< val <<": "
      << std::pow ( std::abs ( val ), 1.0 / 3.0 ) <<std::endl;
  }
}

#include <iostream>
#include <iomanip>
#include <cmath>
 
using namespace std;
 
int main()
{
  double x;
  double a = 1;
  double b;
  char c;
 
  c = ' '; 
 
  while (c != 'q')
  {
    cout << "Cube Root Calculator" << endl;
    cout << endl;
    cout << "Enter a number: ";
    cin >> x;
 
    while (x - (a)(a)(a)) < 0.001)
    {
      a = b;
    }
 
    cout << "Answer is: " << fixed << showpoint << setprecision(5) << a << endl;
    cout << endl;
    cout << "Press c to continue, q to quit" << endl;
    cin >> c;
  }
 
  cout << "Exiting Cube Root Calculator" << endl;
  cout << endl;
 
  return 0;
}

while (x - (a * a * a)) < 0.001)