Delphi：未定义的记录变量

Question

I have defined a TCGTable record type with the following structure:

type
  TCGTable = record
    x : array [1 .. MAX_POINTS] of Single;
    y : array [1 .. MAX_POINTS] of Single;
  end;

I have declared a TCGTable variable CGTable .

The variable CGTable is assigned a particular constant TCGTable record value if that record meets several runtime conditions.

If no constant TCGTable record meets these conditions, CGTable should be undefined.

Is there a Delphi 2010 built-in value I can assign to CGTable to indicate that it is undefined? I have tried using the values nil and null , but both of these seem to be valid only for pointer or variant types. The source will not compile with these values assigned to CGTable .

I would like to inspect the variable CGTable to determine its validity instead of, for example, maintaining some additional boolean validity flag.

The only workarounds I can determine are:

a) Change the type of CGTable to a TCGTable pointer ( CGTable : ^TCGTable; ), which would then allow me to compare CGTable to the nil value.

b) Define some constant TCGTable record to act as an "invalid" record. I would then compare CGTable against this "invalid" record.

Any suggestions on how to approach this? Thanks

Answer 1

You have a record containing a static array . This particular record has a size equal to ElementCount*SizeOf(Element) .

Judging by the the fact that your array is sized with a MAX_POINTS constant it looks like you have a variable number of points in the array. I think I would be inclined to switch to dynamic arrays like this:

type
  TSinglePoint = record
    x: Single;
    y: Single;
  end;
  TSinglePointArray = array of TSinglePoint;

Now if you have a variable, a: TSinglePointArray , then a value of nil indicates that it is empty or nil. You can query for the length of the array with Length(a) . You can resize the array with SetLength(a, NewLength) .

Answer 2

No, your record holds as many bytes as its members take up.[*] If any combination of four bytes is a valid Single, and no more bytes are available for additional information, it becomes impossible to store the fact that the record is invalid somewhere.

You could decide that to mark a TCGTable as invalid, you set CGTable.x[1] to NaN. To check whether it is invalid, you can then check IsNaN(CGTable.x[1]). This only works if all valid TCGTables will have x[1] set to a real value, though. Or you could choose another value that is never valid for x[1]. If there is no such value, you will have to create room for the extra information to be able to check the validity, possibly by storing a pointer to a TCGTable, possibly by adding an IsValid field to your record.

[*] This is not true for all records, but almost definitely is in your case.

Answer 3

If you changed your array types to dynamic arrays then you could check the lengths of them for your invalid record (both at length zero, for instance). Since you're using Delphi 2010 you could also use methods defined in your record to assign values, validate your ranges, and determine if the record is valid.