What's the difference between "var" and "out" parameters?

delphi parameters pass-by-reference
Rob Kennedy picture Rob Kennedy · Jan 24, 2013 · Viewed 19.2k times · Source

What's the difference between parameters declared with var and those declared with out? How does the compiler treat them differently (e.g., by generating different code, or by changing which diagnostics it issues)? Or do the different modifiers merely allow the programmer to document intended use of the parameters? What effect do the types of the parameters have on the matter?

Answer

Mason Wheeler picture Mason Wheeler · Jan 24, 2013

A var parameter will be passed by reference, and that's it.

An out parameter is also passed by reference, but it's assumed that the input value is irrelevant. For managed types, (strings, Interfaces, etc,) the compiler will enforce this, by clearing the variable before the routine begins, equivalent to writing param := nil. For unmanaged types, the compiler implements out identically to var.

Note that the clearing of a managed parameter is performed at the call-site and so the code generated for the function does not vary with out or var parameters.

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