StrPtr New Function for me??? understanding return value

[WFadloun]

Hello all

I searched the forum for detecting when the user press cancel button on inputbox.
I found that when user press cancel we can detect that by using the following function

StrPtr(myinput)=0

StrPtr New Function for me
it works great but still wondering what does StrPtr return value is

thanks in advance
regards

 

[LPlates]

Here's some unofficial documentation on StrPtr, but its main advantage in this case is telling whether or not the return of the input value is "" or vbNullString... taken from the url...

StrPtr is also the only way to tell the different between an empty string ("&quot and a null string (vbNullString). StrPtr(vbNullString) returns 0, while StrPtr("&quot is non-zero.

http://www.mvps.org/vb/index2.html?tips/varptr.htm

 

[paulbent]

The StrPtr function returns a pointer to a string.

A string variable occupies 4 bytes and contains a pointer to the location of the actual string in memory. The variable is initialized to a null pointer, ie 0.

If the Cancel button is pressed, the InputBox function doesn't assign anything to the string variable and StrPtr continues to return 0. When OK is pressed, something is assigned to the variable by InputBox even if it's an empty string and StrPtr will return a non-zero value.

I think StrPtr was mainly introduced to provide support for calling API functions with parameters that require a pointer rather than the string itself. I've never quite understood why it's needed in this situation because one can pass the variable by reference to pass the pointer but no doubt someone will enlighten me!

Paul Bent
Northwind IT Systems

http://www.northwindit.co.uk

 

[strongm]

See thread222-558844, where I try and cast a litle more light on VB strings and StrPtr

 

[WFadloun]

thanks all
very good information
Strongm thanks Thread222-558844 was very helpfull

regards

 

[Hypetia]

>>one can pass the variable by reference to pass the pointer...

Strings are never passed by reference to API functions. The reason is strings variables are itself pointers to data and if you pass a pointer to string, you actually pass a pointer-to-pointer to a string which leads to unexpected results (mostly crashes) when done with API.

I learned this lesson a long time ago when I was trying to write a registry access ActiveX component for Visual Basic and repeatedly crashing my VBIDE. The reason was same. The string was supposed to be filled by the calling API function and I was passing it by reference. I was not aware of this fact at that time and kept knocking my head for a long time.

In VB, the (variable-length) string is a 32-bit variable which points to the location or contains the address of actual string data. When dealing with APIs, we are not interested with the location of this variable but the contents of this variable which is the address of the actual string data (located somewhere else).

If the string is passed by reference of if we pass the value returned by VarPtr function, the location of the string variable is passed to the function which is useless. On the other hand, if the string is passed by value or if we pass the value returned by StrPtr, the contents of this string variable are passed which is the right thing needed by the API function.

Note that the StrPtr function which returns the "contents" or the "address value" stored in a string variable can also be found using the VarPtr function. See the code below.
___
[tt]
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
Private Sub Form_Load()
Dim S As String, L As Long

S = "initialize the string."

'get a pointer to string
'using simple StrPtr function.
Debug.Print StrPtr(S)

'get a pointer to string using VarPtr function.
'copy four bytes from the location of S to L.
CopyMemory L, ByVal VarPtr(S), 4
Debug.Print L

Unload Me
End Sub[/tt]
___

Last but not least is an excellent and comprehensive article from MSDN which explains the storage and management of strings in VB and the way VB interacts with Windows API when passing string data.

http://msdn.microsoft.com/library/en-us/dnw32dev/html/ora_apiprog6_topic1.asp