I'm using Delphi 5. I define an array of byte, as follows:
Buffer: Array of BYTE;
How to convert it to a PByte pointer?
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2 Answers
A dynamic array is implemented as a pointer, so you can simply type-cast it as-is:
var
Buffer: array of Byte;
P: PByte;
begin
SetLength(Buffer, ...);
P := PByte(Buffer);
...
end;
If you don't want to rely on this implementation detail, you can instead take the memory address of the 1st byte in the array:
P := @Buffer[0];
2 Comments
Andreas Rejbrand
But beware of the possible range-check exception if
Buffer[0] doesn't exist.
alancc
Thank you. I modify my question to describe the case in more details. If I use PByte(Buffer) in MyFun, I will get compiler error "Invalid cast". With PByte(@Buffer[0]) is OK. Why?
Two ways to do this:
var
P: PByte;
....
P := @Buffer[0]; // no cast, compiler can check type safety
P := PByte(Buffer); // implementation detail, dynamic array variable is pointer to first element
Note that the former leads to a range error if range checking is enabled and the array is empty. Because of this I tend to prefer the latter, although you may criticise the direct pointer cast for lacking type safety.
answered May 21, 2019 at 7:13
David Heffernan
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6 Comments
alancc
Thanks. But I have two questions: 1) I remember the first byte in the array is the count of the items, so if I use PByte(Buffer), will the pointer actually point to the counter, instead of the first element? 2) I try to use Inspect in Delphi to see (@Buffer[0]) and get a correct address, however, when I use PByte(Buffer), I get "Expression Error", why?
David Heffernan
First byte is not the counter. You are thinking of a shortstring. No compiler error here with the cast. The debugger may not be able to evaluate it but there's not much you can do about that short of upgrading.
alancc
Thank you. I modify my question to describe the case in more details. If I use PByte(Buffer) in MyFun, I will get compiler error "Invalid cast". With PByte(@Buffer[0]) is OK.
David Heffernan
I rolled back the question. You asked originally about a dynamic array. However, your edit talks about an open array. These are very different things, and so I rolled back the edit because it's too late to change the question after you have two answers. However, for an open array you have to use
@Buffer[0].alancc
OK. I see. Thank you very much. Originally I think they are the same thing so I just write a short version of my question. Sorry about that.
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