If I do this:

Dim joe as Guy

This is a reference. In order to use joe I must also do this:

Dim joe as New Guy

This does what's called instantiate the object. What is the difference? Why do I have to do both before I can use the object? Are there reasons why I would only do the reference and not follow it up with instantiate? What really is going on inside the computer when I do each step? When is memory actually allocated?

Thanks. I'm new at object oriented programming. I am used to more procedural languages like COBOL.

The reference created in the first example is indeed a reference, but it's empty. At that point joe is still null. This is useful if you are going to get the actual object from somewhere else and don't need to create a new one. Say, if you had a method that created a Guy object, you would do like so:

Dim joe as Guy
joe = GuyMethod()

which is also the same as:

dim joe as guy = GuyMethod()

Really, you don't need to create a reference and instantiate it on different lines.

This:

Dim joe as New Guy

is the same as:

Dim joe as Guy
joe = new Guy

If you did both lines like you had in your example:

Dim Joe as Guy
Dim Joe as New Guy

You would get a compiler error because you told it to create two separate variables with the same name.

EDIT: On the subject of learning, you might check out this search and this search on MSDN. Some of the articles are a bit old, but the principles are sound.

I think it's funny that when you include the language you want to use, you get a whole bunch of people obviously looking for people to do their homework for them.

Also do you have a example of a method that would perform the instantiate? What would that look like?

Thanks for your help.

10 minutes ago,pdcjlw1 wrote

So at the point that I create the reference what occurs in the computer. Is there a variable created with no memory assigned?

The reference takes up memory, too.  It's value is the 'where in memory is my variable located'.

Think of all variables as 2 things, a location and a value. 

Also do you have a example of a method that would perform the instantiate? What would that look like?

Thanks for your help.

Dim joe as Guy = New Guy()

or, to break it up:

Dim joe as Guy

joe = New Guy()

An example of a method that instantiates a variable is somewhat simple. First you create the reference in your main class.

Dim joe as Guy

Then you have a method that creates a Guy object. Maybe it gets the data from a database or some other source:

Function GetGuy()
    Dim tempGuy as New Guy()
    tempGuy.Name = "Fred"
    tempGuy.Age = 42
    tempGuy.IsAwesome = True

    return tempGuy
end Function

Then in your application somewhere you can put the result of that method into your empty reference like so:

joe = GetGuy()

Remember that VB can both create a reference and fill it in one line like this:

Dim joe as Guy = GetGuy()

EDIT: davewill pointed out that in VB only functions return values. I've edited the code for clarity.

Dim Joe as Something

creates this in memory

Joe ->

Dim Joe as New Something

creates this is memory

Joe -> instance of Something

Dim Joe as Something
Dim Fred as New Something
Joe = Fred

creates this:

Joe -> instance of Something <- Fred

Both references are pointing to the same instance.

Function GetGuy() as Guy
    Dim tempGuy as New Guy
    tempGuy.Name = "Fred"
    tempGuy.Age = 42
    tempGuy.IsAwesome = True
    return tempGuy
end function

Dim Joe as Guy
'Joe is nothing at this point

Dim Fred as New Guy
'Fred is something, a newly initialized Guy (blank name, 0 age, not awesome)

Joe = Fred
'Now Joe points to the same thing as Fred

Joe = GetGuy()
'Now Joe points to a new instance of Guy (a initialized 42 year old awesome guy named Fred)
'Fred still points to his original initialized Guy (blank name, 0 age, not awesome)

Function GetGuy() as Guy
    Dim tempGuy as New Guy
    tempGuy.Name = "Fred"
    tempGuy.Age = 42
    tempGuy.IsAwesome = True
    return tempGuy
End Function


Private Class Guy
  'Property syntax shortened for brevity
  Public Property Name() as String
  Public Property Age() as Integer
  Public Property IsAwesome() as Boolean
End Class

30 minutes ago,davewill wrote

@kettch: one minor edit.  Subs don't return values in VB.

1 2 3 4 5 6 7

Function GetGuy() as Guy     DimtempGuy asNew Guy     tempGuy.Name ="Fred"     tempGuy.Age = 42     tempGuy.IsAwesome =True     return tempGuy end function

Hah! You caught me. I've been working in c# for so long that the context switch defeated me.

Take a look at line two of the code in my quote of your comment. Does it look like the forum stripped out some spaces? I've seen that in other posts in the past few days, where a comment has had random spaces removed. Weird.

4 minutes ago,davewill wrote

@kettch: sure enough.  that's odd.  even odder that the human brain can subconsciously compensate.  i didn't even notice that until you pointed it out.

I just went through the last few threads I've participated in, and the only quote boxes that have this problems are instances where people have quoted somethingI said.

Curiouser and curiouser...

6 minutes ago,kettch wrote

*snip*

I just went through the last few threads I've participated in, and the only quote boxes that have this problems are instances where people have quoted somethingI said.

Curiouser and curiouser...

Yargh! When I hit edit, there is definitely a space between "something" and "I"