where does VarPtr point to?
BlitzMax Forums/BlitzMax Programming/where does VarPtr point to?
| ||
| Hello! Lets assume that I have two variables StringVar:String ArrayVar:Byte[] In that case, will VarPtr StringVar VarPtr ArrayVar point to the first character in StringVar or the first element in ArrayVar, resp.? Both types are actually Objects, which means that VarPtr could yield the address of s.th. else... (FYI: I'm asking because I'll have to pass pointers to external C functions and would like to know if I'll have to do some addtional preparations or may pass the VarPtr's as such - don't worry about "length" issues: the "length" is always passed separately) |
| ||
| Hmmm, I think I found a different approach to solve the underlying problem - but (for academic reasons) the question persists... |
| ||
| Varptr gives you the pointer to the variable itself, like & in c. Passing the array would suffice as its just a pointer to its first element anyway. Strings has to be passed via a $z parameter or by using .ToCString() (dont forget to MemFree() it when your done!). |
| ||
| If you had an int, Local x:int = 5 and you had a C function - void myFunc( int x )
Extern
Function myFunc( x:Int )
End Extern
you could pass x into the function with myFunc(x) If the C function was - void myFunc( int * x )
Extern
Function myFunc( x:Int Ptr )
End Extern
you could pass it into the function with myFunc( Varptr x) For Strings... it depends on the context. I've found occasions where I can define my Extern as myString:Byte Ptr, and simply pass the string into the function itself. In other places I convert to a CString first... |
| ||
| What they said though if the C functions use wide chars use WString instead of CString. |
| ||
| Thanks for your posts! Indeed, a Byte[] seems to be the same as a pointer to its first element - the following test illustrates this: local Text:Byte[] = [byte(0),byte(1),byte(2),byte(3),byte(4),byte(5),byte(6),byte(7)] local TextPtr:Byte Ptr = Text print(TextPtr[0]+","+TextPtr[1]+","+TextPtr[2]+","+TextPtr[3]) print(TextPtr[4]+","+TextPtr[5]+","+TextPtr[6]+","+TextPtr[7]) yields 0,1,2,3 4,5,6,7 as expected. On the other side, a similar code local Text:String = "abc~0def" local TextPtr:Byte Ptr = Text is rejected by the compiler - and local Text:String = "abc~0def" local TextPtr:Byte Ptr = VarPtr Text print(TextPtr[0]+","+TextPtr[1]+","+TextPtr[2]+","+TextPtr[3]) print(TextPtr[4]+","+TextPtr[5]+","+TextPtr[6]+","+TextPtr[7]) produces 16,211,102,0 104,255,168,0 which is *not* what you would expect. |
| ||
| A string is a string. It is a special object no array of char like in C nor is a VAR an actual pointer that you can use on arrays like in C. BM is managed and can if needed move and resort data where needed so if you use pointer, always be aware that they will cause you many problems of done within BM or if you rely on "long term aliveness". If you need array of char, you have to use the above mentioned CString and WString functions and use them. |
   |