Please explain how the sum in the second expression is valid? Thanks

Question

#include <iostream>
using namespace std;

int sum(int a, int b=42) {
int result = a + b;
return (result);
}

int main() {
int x = 24;
int y = 36;

//calling the function with both parameters
int result = sum(x, y);
cout << result << endl;

//calling the function without b
result = sum(x);
cout << result << endl;

return 0;
}

Answer 1

first call to sum returns 60 obviously
in the second call since you're only passing in one argument, default value (42) will be used for second argument and it returns 66

Answer 2

i understand but where is it stated that b is the default parameter?

Answer 3

sum() function is defined with a default parameter b=42. if you don't pass a second argument to the function 42 will be used.

Answer 4

//calling the function without b
result = sum(x);
cout << result << endl;

here how does the compiler recognises - the first expression to be used for sum calculation, when it has not been mentioned in the second expression.

like , sum(x) only states sum of x and nothing more.