simple question

  • Thread starter Thread starter Carly
  • Start date Start date
C

Carly

Hi there,

I know that string is a reference type.

I have the following code:

Dim s As String
Dim s2 As String
s = "AAAAA"
s2 = s
MsgBox(s2)
s = "BBBBB"
MsgBox(s2)

I am not sure I understand why the second MSGBOX still shows "AAAAA"
since the two strings point to the same memory location.

Please help.

Carly
 
Visual Basic treats strings as a special case among the reference types.
The statement "s2 = s" makes a distinct immutable copy of the original string.
 
Thank you Tim

Tim said:
Visual Basic treats strings as a special case among the reference types.
The statement "s2 = s" makes a distinct immutable copy of the original string.
Click to expand...
 
I thought when you do this:

Dim s as String
Dim s2 as String
s = "abcde"
s2 = s
Console.WriteLine(s is s2) --> True

that the compiler sets s2 and s to point to the same place in
memory, and it's only when you assign something different to one
of them that a new reference to a different string in memory
is created.

And in fact, if you set two strings to the same value, like this:

s = "abcdefgh"
s2 = "abcd" & "efgh"
Console.WriteLine(s is s2) --> True

I thought that the compiler will recognize that they are the
same and allocate only one block of memory for the string,
with the two variables pointing to it.

Did I learn something incorrectly?

Thanks,
Robin S.
--------------------------
 
I thought when you do this:

Dim s as String
Dim s2 as String
s = "abcde"
s2 = s
Console.WriteLine(s is s2) --> True

that the compiler sets s2 and s to point to the same place in
memory, and it's only when you assign something different to one
of them that a new reference to a different string in memory
is created.

And in fact, if you set two strings to the same value, like this:

s = "abcdefgh"
s2 = "abcd" & "efgh"
Console.WriteLine(s is s2) --> True

I thought that the compiler will recognize that they are the
same and allocate only one block of memory for the string,
with the two variables pointing to it.

Did I learn something incorrectly?

Thanks,
Robin S.
Click to expand...

Robin,

You are correct - it's called interning. The runtime treats strings
differently then other reference types. Basically, the compiler builds a
table of all the litteral strings in your app and then the runtime will reuse
those addresses rather then allocate new string space each time.
 
Tom Shelton said:
Robin,

You are correct - it's called interning. The runtime treats strings
differently then other reference types. Basically, the compiler builds a
table of all the litteral strings in your app and then the runtime will
reuse
those addresses rather then allocate new string space each time.
Click to expand...

Thanks. It's nice to know I can remember something that I think
is sort of obscure. :-)

Robin S.
 
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