Declaration specifier to explicitly pass a parameter by value

Byval param as datatype

[ declare ] { sub | function } proc_name ( Byval param as datatype )

Byval in a parameter list of a declare statement causes a copy of the variable to be passed to the procedure (for example, a sub or function) by its value.

This means that if the value of the variable x is passed, then the original variable x will not be modified in any way; however, if the variable were passed Byref, the value of the original variable x could be modified by the called function.

Opposite of ByRef.

The Byval keyword is also used in the context of Byref parameters and function results, where it can be used to explicitly override the by-reference semantics in order to pass or assign a pointer as-is to a Byref parameter or function result. For reference:

Note: Byval (passing by copy) is good for small objects (4 or 8 bytes, a bit more will still be ok) that are easy to copy, like numeric types. Passing Byval avoids the overhead of the pointer used by ByRef.
ByRef is better for passing huge objects like strings or big UDTs that should not be copied. Even though ByRef has some overhead because it has to pass a pointer (and dereference at each access to the object), this is still better than copying a multitude of bytes on to the stack every-time the procedure is called.

Sub MySub(ByVal value As Integer)
    value += 1
End Sub

Dim MyVar As Integer

MyVar = 1
Print "MyVar: "; MyVar 'output = 1
MySub MyVar
Print "MyVar: "; MyVar 'output = 1, because byval won't change the values passed into it globally.

Dialect Differences:
Differences from QB:
See also:
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