C++ what happens when toupper is applied to an upper case code example

Example 1: toupper c++

int result = toupper(charecterVariable);// return the int that corresponding upper case char
//if there is none then it will return the int for the original input.
//can convert int to char after
char result2 = (char)toupper(variableChar);

Example 2: touppercase c++

#include <cctype>
#include <iostream>
#include <cstring>
#include <cstdio>

using namespace std;

int main()
{
    char str[] = "John is from USA.";

    cout << "The uppercase version of \"" << str << "\" is " << endl;

    for (int i=0; i<strlen(str); i++)
        putchar(toupper(str[i]));
    
    return 0;
}