can a function be in function definition in c++ code example

Example: function declerations in C++

#include <iostream>
#include <stdlib.h>
#include <iomanip>
#include <fstream>
using namespace std;

void function_one(double, double, double);

int main() {
  double r1 = 1.0;
  double r2 = 2.0;
  double x = 0.0;
  function_one(r1, r2, x);
  return 0;
}

void function_one(double rmin, double rmax, double x0) {
  cout << "Function got called" << endl;
}