What is Parameter Passing and Returning values.. in Programming.. preferbly Visual Basic?

Answer 1

Parameters are the variables you pass to functions. Return values are the variables that are returned by those functions. Values can also be returned by the parameters themselves (such parameters are said to be output parameters).

The variables you pass and return from functions will either be by value or by reference. Only references can be output parameters, since passing by value creates an automatic copy of the value which falls from scope when the function returns. Pointer variables are always passed by value. To pass a reference to a pointer you must pass a pointer to the pointer, which is itself passed by value, but refers to the pointer.

Constant references are the preferred method of passing variables to functions, since there is no need to copy the variable you pass and because it is constant, the original value is left unaltered. Non-constant reference is the next best method, but only if you expect changes to be reflected back in your value (an output parameter). If you do not expect changes, then you must pass by value or, when that isn't an option, create a copy of your value and pass by non-constant reference. The only difference is that passing by value automatically destroys the copy when the function returns.

Ignoring output parameters, a function can only return, at most, one value. A function that returns void has no return value, however it is good practice to always return something, even if only an integer to indicate whether the function was successful (zero) or not (non-zero). If the function is a logical function, such as Add( X, Y ), or an accessor, such as GetName() then it makes more sense to return the actual result of the function via the return value. If you need more than one return value, then you must use output parameters, passed by reference, which allows the return value to be used as an error indicator.

Generally you will want to store the return value of a function, especially if the return value is a memory allocation. If you don't store the return value, the value is lost forever, unless you re-call the function with the same parameters, which is highly inefficient. However, if you only need to know the result of a function once, you don't need to store it, so long as you act upon it immediately, usually as part of a conditional expression. E.g.,

int x, y; // Uninitialised, values unknown.

if( x + y == 12 ) // call the int::operator+ function.

// x+y is definitely 12.

else

// x+y is definitely not 12, but the actual sum is no longer known at this point.