[fpc-devel] About typecasts and the documentation
lazarus at mfriebe.de
Sat Feb 8 23:14:47 CET 2014
On 08/02/2014 21:59, Hans-Peter Diettrich wrote:
> Typecasts can be further restricted to *compatible* types. Here
> numeric types seem to be compatible with other numeric types, but not
> with structured types (records...).
Well than that needs to be added to the docs. I have no problems with
it ont working, I do not need it. What I do is, I do need to know what
works, and how. And I need to know the rule behind.
Besides record and number are not incompatible:
TRecordType(IntVariableOfSameSize) does work.
> It's not clear to me why a TStrings.Objects[i]:TObject can be
> compatible with e.g. integer:
> This may be due to some underlying implementation detail, where the
> list (array) contains pointers instead of objects, and these pointers
> then are compatible with numbers.
Because internally the TObject "is a pointer" . (well that is not
correctly worded..., but a variable of type TObject internally holds a
>> foo := TFoo(longint(1)); // project1.lpr(9,8) Error: Illegal type
>> conversion: "LongInt" to "TFoo"
>> foo := TFoo(1); // project1.lpr(10,8) Error: Illegal type conversion:
>> "LongInt" to "TFoo"
> Obviously the types (record and numeric) are considered incompatible
> by the compiler.
Only if the number is a constant.
But a practical example:
type TFoo = packed record hi,lo: Byte; end;
var a: word;
if TFoo(a).hi = 1 then ; // that works
if TFoo($0101).hi = 1 then; // that does not
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