{"id":1550,"date":"2023-02-26T21:11:50","date_gmt":"2023-02-27T04:11:50","guid":{"rendered":"https:\/\/www.great-white-software.com\/blog\/?p=1550"},"modified":"2023-02-26T21:11:50","modified_gmt":"2023-02-27T04:11:50","slug":"byref-and-byval-part-ii","status":"publish","type":"post","link":"https:\/\/www.great-white-software.com\/blog\/2023\/02\/26\/byref-and-byval-part-ii\/","title":{"rendered":"Byref and ByVal part II"},"content":{"rendered":"\n

Long ago I wrote about byref & byval<\/a>
Apparently this wasnt widely read not even by some long standing members of the community as I still see completely wrong explanations of bevel & byref persist.<\/p>\n\n\n\n

First you have to understand that there are two types of VARIABLES.
VALUE types and REFERENCE types.
With a VALUE type the compiler sees there is space in memory set aside when your program runs to hold a value – a number a floating point value etc. The thing IN that spot IS the actual data.
With a REFERENCE type whats in that spot isnt the data directly – it s the address of the data.<\/p>\n\n\n\n

Normally when you pass a value you pass it BYVAL
This is the default<\/p>\n\n\n\n

So what exactly does this mean ?<\/p>\n\n\n\n

I literally means that the value that is held by whatever type of variable is passed in can’t be altered.
If you pass in an integer, IN the method you can change it however you want, but those changes WILL NOT be preserved in the calling code.<\/p>\n\n\n\n

Something like<\/p>\n\n\n\n

Sub Opening\n  dim I as integer = 100\n\n  system.debuglog str(i) \/\/ prints 100\n\n  foo(i)\n\n  system.debuglog str(i) \/\/ prints 100\nend sub\n\nsub foo ( I as integer )\n  I = 500\n  system.debuglog \"in foo \" + str(i) \/\/ prints in foo 500\nend sub\n<\/code><\/pre>\n\n\n\n
Would print 100 and 500 as indicated. The value would not be preserved from the call to foo. So when we return the next time I is printed in the caller it prints 100.<\/p>\n\n\n\n
This is the easy case for a VALUE type.<\/p>\n\n\n\n
If we switch to a REFERENCE type its EXACTLY THE SAME ! The trick is “the value” that is preserved is the pointer, or reference, that is preserved. NOT THE data referred to. Let me illustrate
<\/p>\n\n\n\n
Sub Opening\n  Dim d As New date\n  System.debuglog d.SQLDateTime\n  foo(d)\n  System.debuglog d.SQLDateTime\n\nEnd Sub\n\nsub foo ( d as date )\n  \n  d = New date(d.year - 2, d.month - 4, d.day - 6, _\n    d.hour - 8, d.minute - 10)\n\n  System.debuglog \"in foo = \" + d.SQLDateTime\nend sub\n<\/code><\/pre>\n\n\n\n
Note that if you run this you will see that d in the main calling routine before & after the call prints the SAME date. This despite the fact that in the called routine, foo, we altered the date that d referred to.
What this shows is that what is being passed by VALUE is the REFERENCE (pointer or address)<\/p>\n\n\n\n
And again IN the routine we called we can do whatever we want to change what the REFERENCE is by creating a new one and that change wont stick when we leave.<\/p>\n\n\n\n