On Sun, Nov 2, 2014 at 3:54 PM, Xiangrong Fang <xrf...@gmail.com> wrote:
> Also, I usually use pointer to pass block of memory to functions. How do > I implement a function with the following signature: > > procedure MyProc(var Buf; Len: Integer): > > I mean, how to handle "var Buf" inside the procedure body? > Use "@Buf" to get a pointer to the address of the untyped variable. If you want to cast that variable to an internal "known" type and the compiler complains you can always use a pointer cast: PMyKnownType(@Buf)^. You can also use pointer arithmetic (e.g. (PElementType(@Buf) + n)^) if the variable is known to be an array. In Delphi (and probably FPC) it is allowed to pass a nil pointer to an untyped variable if you need to: begin MyProc(nil^, 0); end; The compiler understands what you are trying to do and does not complain about the blatant nil dereference. So "var Buf;" is functionally equivalent to "Buf: Pointer;". OTOH "const Buf;" is slightly different in the sense that the compiler will not let you pass the untyped variable to another routine that may change its value. -- Constantine
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