In call by value, a parameter acts within the function as a new local variable initialized to the value of the argument (a local (isolated) copy of the argument). In call by reference, the argument variable supplied by the caller can be affected by actions within the called function.
Call by Value
Within most current programming languages, parameters are passed by value by default, with the argument as a copy of the calling value. Arguments are isolated, and functions are free to make changes to parameter values without any risk of impact to the calling function. Consider the following pseudocode:
Function Main Declare Real fahrenheit Assign fahrenheit = 100 Output "Main fahrenheit = " & fahrenheit Call ChangeFahrenheit(fahrenheit) Output "Main fahrenheit = " & fahrenheit End Function ChangeFahrenheit (Real fahrenheit) Output "ChangeFahrenheit fahrenheit = " & fahrenheit Assign fahrenheit = 0 Output "ChangeFahrenheit fahrenheit = " & fahrenheit End
Main fahrenheit = 100 ChangeFahrenheit fahrenheit = 100 ChangeFahrenheit fahrenheit = 0 Main fahrenheit = 100
In English, the Main function assigns the value 100 to the variable fahrenheit, displays that value, and then calls ChangeFahrenheit passing a copy of that value. The called function displays the argument, changes it, and displays it again. Execution returns to the calling function, and Main displays the value of the original variable. With call by value, the variable fahrenheit in the calling function and the parameter fahrenheit in the called function refer to different memory addresses, and the called function cannot change the value of the variable in the calling function.
Call by Reference
If a programming language uses or supports call by reference, the variable in the calling function and the parameter in the called function refer to the same memory address, and the called function may change the value of the variable in the calling function. Using the same code example as above, call by reference output would change to:
Main fahrenheit = 100 ChangeFahrenheit fahrenheit = 100 ChangeFahrenheit fahrenheit = 0 Main fahrenheit = 0
Programming languages that support both call by value and call by reference use some type of key word or symbol to indicate which parameter passing method is being used.
|Language||Call By Value||Call by Reference|
|Java||default||applies to arrays and objects|
|Python||default||applies to arrays (lists) and mutable objects|
Arrays and objects are covered in later chapters.
- call by reference
- Parameters passed by calling functions may be modified by called functions.
- call by value
- Parameters passed by calling functions cannot be modified by called functions.