Before learning how to pass a variable by reference, we will review the concept of passing by

value, which you learned about in Chapter $9.$

Reviewing Passing Variables by Value

Recall that unless you specify otherwise, variables are passed by value in C$++.$ This means that

the computer passes only a copy of the variable$$s contents to the receiving function. When only

a copy of the contents is passed, the receiving function is not given access to the variable in

memory, and, therefore, it cannot change the value stored inside of the variable. It is appropriate

to pass a variable by value when the receiving function needs to know the variable$$s contents but

does not need to change the contents.

The company ratings program shown in Figure $10-8$ passes a variable by value to a void

function named displayRating. The function definition is located below the main function

$($on Lines $28$ through $34).$ Therefore, the program includes an appropriate function prototype

above the main function $($on Line $8).$ Because the displayRating function is a void function,

its function call $($on Line $18)$ appears as a statement by itself. The function call passes the

numStars variable by value to the void function. This means that only a copy of the variable$$s

value is passed to the function, which stores that value in its formal parameter $($num$).$ The

displayRating function does not have access to the numStars variable. It is not even aware

of the variable$$s existence in the computer$$s internal memory.