example for https://www.notion.so/essentialbooks/Function-8ed3177ded73416a960f6580f5937e0b (made with https://codeeval.dev)

function.cc

#include <iostream>

// function declaration
int add2(int i);
// functin overloading: same name but different arguments
int add2(int i, int j);
// default arguments: i is optional, if not given will be 0
int add3(int i = 0);

// function defintion
int add2(int i)       // Data that is passed into (int i) will be referred to by the name i
{                     // while in the function's curly brackets or "scope."
                    
    int j = i + 2;    // Definition of a variable j as the value of i+2.
    return j;         // Returning or, in essence, substitution of j for a function call to
                      // add2.
}

int add2(int i, int j)    // However, when add2() is called with two parameters, the
{                         // code from the initial declaration will be overloaded,
    int k = i + j + 2 ;   // and the code in this declaration will be evaluated
    return k;             // instead.
}

int add3(int i)
{
    return i + 3;
}

int main()
{
    std::cout << "add2(2)   : " << add2(2) << "\n";
    std::cout << "add2(3, 4): " << add2(3, 4) << "\n";
    std::cout << "add3()    : " << add3() << "\n";
    std::cout << "add3(3)   : " << add3(3) << "\n";
    return 0;  
}