Four function calculator using functions in c++.

#include<iostream>
#include<string>using namespace std;
double sum(double a, double b);
double sub(double a, double b);
double mul(double a, double b);
double div(double a, double b);
int main()
{
 double first;
 double second;
 char choice;
 cout << "Enter the first number : " << endl;
 cin >> first;
 cout << "Enter the operation do you want to perform : " << endl;
 cin >> choice;
 cout << "Enter the second number : " << endl;
 cin >> second;
 cout << "The result is : ";
 switch (choice)
 {
 case '+':
  cout << sum(first,second) << endl;
  break;
 case '-':
  cout << sub(first, second) << endl;
  break;
 case '*':
  cout << mul(first, second) << endl;
  break;
 case '/':
  cout << div(first, second) << endl;
  break;
 }
 cout << endl;
  system("pause");
 return 0;
}
double sum(double a, double b) {
 return a + b;
}
double sub(double a, double b) {
 return a - b;
}
double mul(double a, double b) {
 return a*b;
}
double div(double a, double b) {
 return a / b;
}