Question

Explain why pass by value-result needs storage for both the value being passed AND the location of the actual parameter being passed.

Answer 1

pass by value-result is an implementation model for out-mode parameters.When a parameter is passed by result, no value is transmitted to the subprogram. The corresponding formal parameter acts as a local variable, but just before control is transferred, which obviously must be a variable (how would the caller reference the computed result if it were anything else?).If values are returned (as opposed to access paths), as they typically are, pass-by-result also requires the extra storage and the copy operations that are required by pass-by-value. As with pass-by-value, the complexity of implementing pass-by-result prevents transmitting an access path instead of a value.In this case, the problem is in ensuring that the initial value of the actual parameter is not used in the called subprogram.

pass by value-result is an implementation model for inout-mode parameters in which actual values are
moved.It is in effect a combination of pass-by-value and pass-by-result.The value of the actual parameter is used to initialize the corresponding formal parameter, which then acts as a local variable.In fact, pass-by-value-result
formal parameters must have local storage associated with the called subprogram.At subprogram termination, the value of the formal parameter is transmitted back to the actual parameter.

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