Basic OOP question

[ThePentiumGuy]

Hey...

As I went over one of my OOP tutorials which I made like a year ago... I began to notice some funky stuff.

 

Are objects always ByRef?

 

Public Sub New(ByVal target As Form, ByVal YourColor As Color)

target.BackColor = YourColor

End Sub

I mean seriously - I wouldn't expect this to happen unles the form argument is byRef instead of byVal. I noticed this too when you say 'byval sender As Object'.

-The Pentium Guy

[PlausiblyDamp]

Objects are always passed by reference.

[ThePentiumGuy]

Heh, that answers it.. thanks :).

[Nerseus]

To be more exact, you have to talk about a class versus a struct not just "object" which means different things to different people. Also, it's only the data within a class that's passed ByRef. The variable itself can be passed ByRef or ByVal and it does make a difference.

 

For a struct - any value type really - ByVal and ByRef works more the way you'd expect.

 

Here are two threads that talk about it a little more:

http://xtremedotnettalk.com/showthread.php?t=78621

http://xtremedotnettalk.com/showthread.php?t=84175

 

There was a better thread than these two, but my searching isn't what it used to be - I couldn't find it (only tried looking for ByRef).

 

-ner

[mskeel]

I found this out the hard way recently. The MSDN is sort of misleading with regaurds to byref vs byval. I had always assumed that they acted the same as the traditional C++ value and reference passing, copying memory, speed and preformance, by value can't change the parameter...

 

three questions:

 

1. Are structs value types and do they act as one would them to if passed by value or refence? (by value => no changes, by reference => changes)

 

2. Passing byval ensures you can't change the pointer, but is there a way to pass a pointer so that you can't change its contents? Or do you just have to be extremely careful not to change anything? Something equivelent to a const reference pass in C++.

 

3. I assume this is a .Net problem, not just a VB one, yes?

[PlausiblyDamp]

Structs are value types and classes are reference types. Passing a value type by reference allows changes, by value means no changes.

There is nothing in .Net that mimicks the idea of a C++ const reference.

 

Passing a reference type by value means you can change the contents of what the variable points to but not what the variable points to.

Passing a reference type by reference means you can change what the variable actually points to.

 

Find a small app attached which demos these using integers and textboxes for for value types and reference types.

ValuesAndReferences.zip

[mskeel]

And what about static members? Shared, I think in VB...

[PlausiblyDamp]

Shared memebers are not associated with an instance of a class / structure so this doesn't apply to them

[mskeel]

Ahh, that's right. sorry, I forgot how shared works. I don' think you can even declare data members as shared anyway so I think my question regaurding static members is irrelevent anyway.

[PlausiblyDamp]

Data members can be shared - it just means the variable only exists once regardless how many class instances are created.