Which statement describes the effect of passing by value versus passing by reference on the caller's data?

• A. Passing by value copies data; passing by reference allows the function to modify the original data.
• B. Passing by value allows modification of the original data; passing by reference copies the data.
• C. Both pass by value copies data; by reference does not modify data.
• D. By value does not copy data; by reference copies data.

Answer: (A)

The crucial idea is how arguments move between the caller and the function. When you pass by value, a separate copy of the data is made for the function to use, so any changes inside the function affect only that copy and not the original variable in the caller. When you pass by reference, the function works with the actual variable from the caller (the reference to it), so changes inside the function directly modify the caller’s data.

For example, x starts at 5. If a function takes its parameter by value and then assigns 10, x remains 5 after the call. If a function takes its parameter by reference and assigns 10, x becomes 10. This is why the description says: by value copies data, and by reference allows modification of the original data.

Other statements that suggest the opposite—that passing by value lets you modify the caller’s data, or that passing by reference copies the data—misrepresent how these mechanisms pass data between caller and callee.