Passing arguments by value

Code:

---

#include <iostream>
using namespace std;
int add (int i)
{
        i=i+100;
        return i;
}
int main()
{
int n=1;
add(n);
cout << "n:" << n << endl;
n=add(n);
cout << n << endl;
        system("pause");
        return 0;
}

---

Why is result 1 in statement add(n) and why it is 101 in n=add(n).I have also noticed that if I write

Code:

---

cout << "n:" << add(n) << endl;

---

instead

Code:

---

add(n);

---

result is 101?

In which cases it will return 1 and why?

Since the parameter is passed by value, the function cannot modify it. The function modifies a copy, then returns the result. So do this instead:

Code:

---

int n=1;
n = add(n); // Assign n to what the function returns

---

Quote:

---

Originally Posted by bithub

Since the parameter is passed by value, the function cannot modify it. The function modifies a copy, then returns the result. So do this instead:

Code:

---

int n=1;
n = add(n); // Assign n to what the function returns

---

---

But why I need to assign it to n?

Because the add function isn't modifying the passed value. If you want the add function to modify the value that is passed in, then do this:

Code:

---

int add (int& i)
{
        i=i+100;
        return i;
}

---

This way i is passed by reference instead of by value. Now any changes add makes will change the caller's variable.

Code:

---

int n = 1;
add(n);

---

In the case where the variable is passed by value, the above code will change the local version of the variable n (called i within the function itself) however, since this new value (although returned to the caller) is not assigned back to n in main, it (n) maintains its old value of 1.

Code:

---

int n = 1;
n=add(n);

---

In this case, the returned value 101 is assigned back to n overwritting what was there (a 1). Thus n will now contain 101 after the function call.

Code:

---

int n = 1;
cout << "n:" << add(n) << endl;

---

This will print the value returned by the function (101) but will not update n, n will still remain 1 after the call to add because it hasn't been changed.

I understand it now.

Moderators can lock this thread.