What is good manners in c++?

Question

Programming

I watch a video on C++ and the author explains how the functions work and writes the following code for clarity:

#include <iostream>
using namespace std;
int sum(int,int);
int main()
{
  int num1;
  int num2;
  cout << "Введите первое число: ";
  cin >> num1;
  cout << "Введите второе число: ";
  cin >> num2;
  cout << sum(num1,num2) << endl;
  return 0;
}
int sum(int number1, int number2)
{
  return number1 + number2;
}

But the second option will also work:

#include <iostream>
using namespace std;
int sum(int number1, int number2)
{
  return number1 + number2;
}
int main()
{
  int num1;
  int num2;
  cout << "Введите первое число: ";
  cin >> num1;
  cout << "Введите второе число: ";
  cin >> num2;
  cout << sum(num1,num2) << endl;
  return 0;
}

So the question is - is it a good idea to create a function in advance, and then describe it (as in the first option) or is it possible to immediately describe the function (as in the second)?

Answer 1

What exactly is a prototype for? Why can't we limit ourselves to using a single function declaration? The prototype became necessary after the C language standards changed in such a way that before calling a function in a file, it must be declared in some way. The problem is that a function name has a global scope (if its description is outside any local scopes). Let's assume that the description of the function is in a separate source file. Let's also assume that we want to call the same function in several other source files. If there is no prototype, then a full description of the function must be included in each such source file. The compiler will interpret this as an override. If we use a prototype, then we can include this prototype in as many source files as we need.
Taken Here