Pass By Value

Consider a pair of C++ functions defined in Program . The function One calls the function Two. In general, every function call includes a (possibly empty) list of arguments. The arguments specified in a function call are called actual parameters . In this case, there is only one actual parameter--y.

  
Program: Example of Pass-By-Value Parameter Passing

The method by which the parameter is passed to a function is determined by the function definition. In this case, the function Two is defined as accepting a single argument of type int called x. The arguments which appear in a function definition are called formal parameters . If the type of a formal parameter is not a reference (see Section ), then the parameter passing method is pass-by-value.

The semantics of pass-by-value work like this: The effect of the formal parameter definition is to create a local variable of the specified type in the given function. E.g., the function Two has a local variable of type int called x. When the function is called, the values (r-values) of the actual parameters are used to initialize the formal parameters before the body of the function is executed.

Since the formal parameters give rise to local variables, if a new value is assigned to a formal parameter, that value has no effect on the actual parameters. Therefore, the output obtained produced by the function One defined in Program  is:

2
1

Copyright © 1997 by Bruno R. Preiss, P.Eng. All rights reserved.